I submit that the answer (if indeed you buy the variety with bumps/dimples) is determined by a higher, more strategic decision.
Namely, which way do you position the TP roll on the hanger? With the loose end abutting the wall, or the loose end away from the wall?
If the former, then the dimples face you as you tear the strip off. If the latter, the bumps face you as you tear. (Of course, this assumes you always fold in the same way.)
Answer that question, and you have your answer regarding bumps or dimples.
Personally, I use the cat if it happens to be passing at the crucial moment. Such soft and absorbent fur.