While I think you are fine to just tip from the bill, we typically add a little bit. It's pretty common for my traveling party to visit a higher end eatery (say, Flying Fish) and just do apps/dessert - usually because we saw something on the dessert menu we wanted to try, but also because it brings the experience into our price range.
We are allergy diners, which means we take up the table for nearly as long as a full meal, and we have TiW. If the service is good (and I have NEVER experienced pushback from the server even on a water/app/dessert bill - common when it's hot,) we increase our tip - sometimes significantly. That's where my moral compass rests when it comes to taking up a table and ordering the cheapest things on the menu. (Frankly, if we ordered entrees, we'd be too full for dessert - which is often why we went in the first place!)
But Rusty's correct in that it's not necessary. I just feel less like a cheapo by doing it. Lol!
