Interesting. I'd never considered this, and decided to pose it to family members who are servers.
None cared whether a customer tipped to make the total a round number, or tipped a round number. However, one *did* say that, in the case of someone who tips to make the total a round number, if the tip was low (say 10% or less), she - when entering the total check into the POS system - will subtract a penny, to make the total $XX.99, versus $XX.00.
As for tipping in general, I always tip on the total bill, and start at 20%, going up for better service. Using Jaimito Cartero's example, I'd rather the server have the extra $1.35. I've been very fortunate in my life, and know that $1.35 will mean more to the server than it does to me.
Note: this is only for tipping within the US