I tip 15%.
I'm sorry, but as prices go up, I don't see why the percent of the tip has to go up as well. I'll stick with the 15% I've learned.
For excellent service (or if I'm feeling generous), I'll do 20%.
And for takeout, I'll generally tip nothing. If you want a tip, a significant service needs to be provided and "I boxed up your food" doesn't cut it.
Further, I have no idea if the person ringing me up and handing me the bags did anything other than punch numbers into the cash register.
The one exception: for the local mom and pop Vietnamese place we frequent, we do tip on takeout, but not as much as when we are eating in. Since there's only 3 people who ever work there, I'm fairly certain that they provided all the service.