I'm of the mindset that I want to be as scent free as possible but i just don't get too carried away. My dad has a gas powered UTV and we use it all the time. He generally drops me off somewhere near my stand then he'll take it down pretty close to where he wants to hunt, park it and walk in. I've seen deer in the field walk right past where he parked it but I've seen them do that with a truck parked in a field as well. We give deer a lot of credit and I'm not saying the smell of gas/4-wheeler/UTV couldn't spook a deer I'm just not convinced they link every smell to a human threat or danger. I'd rather get dropped off 100 yards out and have an extremely short hike(no sweating) than hike in 500-1000 yds. and have no scent of fuel in the area.
Starting using polaris crew two years ago. Haven't noticed a difference. I spray down once I park. Try to use cover scents in the tree. Seems to work for me. Doesn't mean I haven't spooked a deer I don't know about, but I haven't experienced a drastic difference between a truck and the UTV. I was worried about the same thing you are talking about. Might help being the UTV is a crew. I have no idea. I do think deer are more alert when they see a vehicle that isn't normally there. We park in a spot that I believe sometimes will actually help turn deer back to us. I have seen deer right by both types of vehicles too, but I believe overall it is better to walk in aways from any vehicle.
ive always questioned the same, personally I dont. im still young and a little exercise never hurt nobody, but my uncle is lazy n kills em up every year anywho..
b_walte15 is dead on! If 4 wheelers are rode on the property year round, the deer will get used to them and not really pay them much attention. I have rode past deer that were 50 yards away back in the woods that would just stand there and watch you ride by and not spook. But if they never see a 4 wheeler all year and all of the sudden deer season rolls around and a 4 wheeler comes down the trail, their reaction is going to be different.
Agree with b_walte15 and BearArcher, deer and elk around my cabin are used to hearing us all year, doesn't make much difference. Some of the back country places we go it's really nice to keep the UTV somewhat close.