Adding up what I remember, I come out to a real rough estimate of 30% standing or offhand, another 30% either sitting or kneeling, often using a hasty sling. 30% prone, which is with a sling, off a bipod, or pack. Another 10% of "other", such as shooting out of blinds.
Field expedient rests such as a rock or something are used when available, though a properly adjusted sling and some practice works about as well as anything.
So much depends on what and where a person hunts, and if a person gets around and hunts some different terrain and species without practicing different techniques, it's easy to get outside one's comfort zone.
Overspecialization breeds in weakness, as the old saying goes.
