If the sight or sound of an atv is not common to the area on a year round basis, I guarantee you will spook deer. I've been on stand and witnessed plenty of deer bolt from the sight or sound of an atv where there's not a common presence of them. Deer may move into the area after you get there, which may seem like you're not spooking them, but you've probably blown most of the deer out of the immediate area. Most definitely the mature deer.

If there is a year round presence of atv's, it won't bother them as much, as long as the atv's presence is common during that time of day in that particular location.

Case in point, we drive atv's, on a daily basis, through our farm from my folk's house, to the barns, to my house. We have a gravel road that runs through the farm with a food plot and an alfalfa field right along part of that gravel road. If we're driving on the road, the deer will usually jump a few yards into the woods, some times staying in the fields as close as 40 yards of us as we drive by. But, if you drive off the road, see you later.

FWIW.