I almost always round to the nearest dollar, usually up. I have never, ever had a server complain or even give me the stink eye for leaving, say, a $6.72 tip instead of $7, or as many people would have, $6.

I suppose for those servers who take their tips as cash at the end of the shift, it's a bit more a hassle to count out the change, but if that's your biggest problem in life, you're doing fine.