In searching for your answer, I found this instead:
The Straight Dope: Urinal 101: Aim for the back wall or the water?. I'm easily distracted.
Anyway, (and I have no links to back this up), from what I recall, the Coriolis Force is very very slight. The flush direction of a toilet will be much more affected by the shape of the toilet and the direction of the water jets.
EDIT: Ah, here we go, will you accept the word of
Snopes? They say 'false'.