When you come to take my payment for the bill at the end of a meal, please do not ask me if I need change. When you do that, what you're really asking is, "Can I keep all of this?" And the only conclusion to be drawn if I say that I need change is that I'm cheap.
The bill is $15.05, and I put down a $20. You ask if I need change. Now I'm thinking, "Well, I was going to leave $3 and pocket a dollar bill and a few quarters for the parking meter. But if I say "Yes, I need some change," and then my fellow guests see me scraping three quarters and a dime off the little plastic tray into my grubby little hands, they'll think I'm so cheap I'd begrudge a hard-working waiter three quarters and a dime! So I'd better just say, "No, keep it."
It's not worth a buck to be considered cheap by my friends.
But then again, your service was so-so, and you just Kobayashi Maru'd me into giving you a 25% tip! Eff that! Bring me my damn change!
I don't care if it's more convenient for you to know in advance. Too bad. Bring my change, and I'll decide how much of a tip to leave in private, in my own good time. You should enjoy the suspense. Every table is like a Christmas tree, and every customer is Santa (or Scrooge).
I bet you can't wait to see what we left you!