Me personally, if I have a bill of $7.50 (food delivery, restaurant check, taxi), I'm tipping an even $10. Again, just me personally, I usually round up to a round-ish amount, to the benefit of the person I'm tipping. I don't go nuts and hugely over-tip, but if it's a difference of a buck or 2 I figure I'll give it to the person providing a decent service for me.