20% for me. Unless the server is a dick. Even if service is bad/slow -- still 20%. Maybe it's their first day. Or maybe they're understaffed. Who knows.
Also, at most places, they pool the tips and have to tipout bussers, hosts, and other support staff. So you're really taking away from them as well. I will tip better for good service or someone who goes beyond what they have to do, so I guess in these cases those people are benefitting as well.
Yup. Though I am definitely not in that 35% of people who tip for restaurant carry-out. Definitely will for delivery, though I rarely get anything delivered.
I posted this article in another place and someone made a point that a dollar or two can go along way for the people working in terms of carry-out. Basically, it's a nice gesture, I doubt I'll change my philosophy here. Remember Drew Brees got criticized for his lack of tipping on carry-out, but if I had his kind of money, I'd be a lot more generous than he was. Delivery you should definitely tip.
Something I always make sure to do is tip on the pre-discounted amount and/or if you get something on the house, make sure the tip reflects what it would've been otherwise. I notice at some places, I don't get charged for a soda. When I pointed out this "error" to the bartender a few weeks ago, she told me it wasn't. I just gave her the amount it would've cost because I thought it was a nice gesture on her part.
A bit of a side-note, but the Groupon/Living Social deals are almost always a rip-off for this reason. You do $10 for $20, they make you spend $20 (which usually means you have to go over) including tax it usually gets to around $12-13 then add tip it's $15-16. So you really end up saving $4-5 instead of the $10.