I hunted 500 last September. I have a 04 tacoma and there wasn't anywhere in the unit I went that I felt I would have needed an atv so I'm going with a no.
OnX with the Road/Access type overlay shows roads with Highway vehicles or hiking trails only so the ATV really would not give you an advantage. There are other units however that would benefit greatly from an ATV as Colorado can be strict between ATV and SxS access.