fine on my mac. i don't think i've EVER seen an issue with jb'ing and itunes updates...
Umm, I remember there once was an issue with a particular JB and iTunes version - around the same time people started asking JB devs if iTunes were safe. You'll find lots of threads here on MR.
But then it sort of died out, i.e. nobody's asking and answering any longer. But it's not like this sort of question is totally unsubstantiated.
I think I even remember somebody from Dev Team "clearing" an iTunes relase, but I may be wrong.
All I know is I NEVER update any software unless it has been tested and deemed safe. IIRC, in the past 5+ iTunes releases, MuscleNerd has commented about their safety.