I think a lot of fast food places aren't allowed to accept tips, which explains the high turnover rate. I always tip at any restaurant that is allowed to accept them.

The servers are providing me a service, even if it's just making sure my order is ready quickly, accurately and packaged well enough to make the trip home. I don't tip as much as I would for table service, but generally 10% for takeout (I pick up) and 15% for basic table service, 20% for good service and delivery.

If I plan to frequent a restaurant regularly (it's nearby and the food is good), I tend to think of tipping as an investment and it's usually paid off in excellent, over-the-top service the more I go there. My mom is a notorious bad tipper, and she wonders why she gets all the inexperienced servers who make mistakes. I try to tell her good service is worth paying extra for and she might get more experienced staff or better service if she didn't nitpick the tip down.

I am not now, nor have I ever been a waitress, so this isn't a biased posting.