I think it depends on how much you use them. I used to hunt deep woods deer where any motorized noise would spook deer.
I now hunt on two farms where we use atvs regularly for checking cattle, fixing fences, pruning peach trees, almost daily. The deer don't seem to pay much attention to them at all. I've got pictures that show mature bucks walking a firelane just 10 minutes after I drove down the lane checking fences.