I almost aways tip at least 20% at restaurants.
Which is mostly a result of living the following first hand:
-Most waiters make a very paltry sum hourly from their employer ($2.15 when I was working in NC), with the expectation that they will make at least minimum wage considering tips. This is the minimum requirement by the state, but most employers don't see a reason to pay more.
Employers are required to make up the difference if you don't make minimum wage, but at least for me it was a per paycheck rule and I don't think I ever saw any additional pay because of it. That means, if you worked on the slower weeknights (which most waitstaff were required to for the benefits of a weekend), you could earn only $20 or $30 (which happened a lot during slower times of the year, especially since they cut people early if we weren't busy and scheduled too many people just in case)
-Waiters may be tipping out to their bartenders and hostesses. I think ours was 3% out of our total sales, which means if someone left us no tip at all, we were losing money. If you tipped 15%, 3% of that was not ours. I don't begrudge the hostesses and bartenders making money at all, but paying the hostesses a good hourly wage and letting the restaurant handle the bartender's cut sounds okay to me as well.
-20% is now considered a standard tip. I am sure not everyone has this expectation, but where I worked, we wondered what we had done wrong when we didn't get 20%. I wondered what I did wrong when a foursome came in and spent $90 and tipped me nothing, who wrote in zeroes. I'm assuming that the man who so graciously decided to pay didn't like the way his steak was cooked, although, when I asked if everything came out right, he declined to offer up that information. I know that the internet world doesn't know me, but I am pretty even tempered with customers.
Before I started working at the restaurant (a pretty popular and nice Italian restaurant chain), my dad tipped 15% always. Within a few weeks I had asked him what he tipped and explained what working at the restaurant was like, and he changed his ways, mostly for me. He didn't know it had changed. Some people feel like 15% is standard and 20% is if they wowed you.
-You aren't teaching anyone a lesson when you tip them less than 20%. You had an awful server and you decide you are doing them a favor by leaving them a small tip. Now they will know that they did a bad job and improve in the future. Actually, they will consider you to be an asshole, and if they see you again they will pray you aren't their table. If they do think they've done a bad job they will feel bad, but probably not improve. And chances are they don't think they did a bad job. Or, it might have been the kitchen's fault. Or, this is their second job of the day and they are tired and miserable. Or, they are new. It really doesn't matter. Tipping only really means "good job" when it's not expected. Take it up with restaurant owners if you don't like it, or stop eating out. You won't, because it doesn't affect you.
-You should factor in tips before you make the decision to eat out. Restaurant owners care if you tip, but not as much as they care if you eat out. They want everyone to feel like they can afford eating out. However, if you decide to really treat yourself and end up with a $100 check, I hope you aren't thinking $10 is a good tip. That anyone would be happy with $10 whole dollars from just one table. They aren't. They are giving three of those dollars to tipshare. You were probably taking up their table for at least an hour. You might be their only table or you might have been really demanding when they had several other tables to deal with.
-Waiters come in before their shift starts and stay after. Where I worked, at the end of every night we were required to clean our tables, "vacuum" around them, refill the sweeteners and refill the olive oil. Then, we had a job to do: clean out the tea urns, polish the wine glasses, clean the bread cutter, etc. During the shift we had "running duties" such as helping the bartender restock or being responsible for starting a new batch of coffee or tea. We had to wait on someone to check us out at the end of the night. We had to wait for our last table to leave before we could do any of that. Some of us were so desperate to get out of the restaurant that we'd pay another server to do our jobs and take over our tables if they were closing. You don't get paid extra to stay late. You get paid $2.15 an hour. Closing only paid off if people preferred paying you to doing their jobs or if you got extra tables after some waiters had gone home. Opening meant that you got seated first, and usually you got a good section.
-When you come to a restaurant to catch up with old friends or just to order dessert and coffee but sit for 2 hours and tip based on the check, you are killing your waiter with a slow painful death. You are either unaware or inconsiderate. I've heard people use the excuse that it's not very busy so it doesn't matter. Where I worked you only started out with two tables until you could be trusted with three, and on weekends often we only had two anyway, because they wanted to make sure we did a good job with each table. If both of my tables were full and both parties were enjoying themselves after a meal or taking their time with the large salad they split and their two waters, I was stuck.
I've been really uncomfortable as a customer when I'm out with family or friends and we've been there a long time. I've tipped extra after covertly seeing what a friend tipped (when we hadn't stayed long after the meal). I've squirmed when someone ordered another coffee. I can't help but feel for these servers, even incompetent ones, because I've been there. I round up on tips, it makes the math easier. I never tip less than 20% to make it a whole number or tip on the pre-tax amount (which I'm not necessarily knocking).
I do feel like I could write a book on the topic, it is what waiters breathe every day, mutter to each other after every table. The amazing tips aren't as frequent as you'd like. The small tips make you feel crummy and the non-tips make you livid. The car salesman-like waiters that work with you and make great tips but also pull out photos of their children and say it's their birthday to get better tips make you sick. You worry about pushing dessert because while it ups their tab, they might be dead set on only tipping you $5 no matter what, and especially if you make them pay more on their check. You are ticked off when it takes the kitchen 30 minutes to get your order out because you know the table will be there longer which means less turnover, and will be less pleased which means less tip.
I've gone on for too long, and I don't expect anyone to actually read all of it, but hopefully the bolded text sticks out to some people. I want people to be informed when they vote and when they eat out.