I'm not sure if the temperature has any effect, but I do know many people have voiced problems with the "rack on top" method, claiming uneven carbing due to the priming sugar not being totally mixed in.
The solution to this is to very gently stir the beer a little after racking onto the priming sugar solution. And again, this is going to be one of those issues where you just have to choose a side. Some feel the possibility of oxidation is too great, even with a gentle stir, and believe that racking on top of the sugar solution is more than enough to mix it up (despite temperature). Others feel that a gentle stir doesn't introduce anywhere near the amount of oxygen to cause concern, and is better than ending up with uneven carbonation.
I've done it both ways, and here's what I've found:
When I didn't stir, the bottles carbed fine. When I did stir, there was no noticeable off flavors from oxidation. In my book, either way you go you'll probably be fine.
Personally, I usually give a gentle stir to be safe. But if I forget (which I do now and then) I don't fret over it.