Downloading any software package is safe, it's the installing which might cause problems.
However, downgrading software on a computer is always possible.
The updated by accident is an iPhone firmware problem, not iTunes problem, since one can always revert to a former version of iTunes.
And iTunes 10 is safe as proved by the replies in the dozen or so existing threads about this (before yours).
Of course one could also fallow Dev-Team members on Twitter since they always report what's safe and what's not. Musclenerd being the best one to fallow IMO but I fallow several members.