When using a credit card I always make the total a round $$ amount. I do this for 2 reasons.
1. It helps prevent fraudulent use of my card, if I see a charge that is not a round number I know its a mistake or fraud.
2. As most others have said it gives the server that little extra as a tip. I tend to calculate the tip on a dollar amount then round up with the change to make a total bill round number.