Consistency is everything. Everything. Sight alignment, trigger squeeze, and grip position and tension, all require uniformity. While .22's can be shot consistently with a soft grip, big bores / heavy recoilers will require a firm (but not trembling) grip with the shooting hand. I simply support that tightly gripped shooting hand in my off (left) hand without applying pressure with that off-hand, but I have known at least one very good shooter who applies pressure with both hands. My counsel is a tight grip with the shooting hand, rested in the grip of the support hand, beside it. I've been shooting so long, I rest the support hand forefinger against the front of the trigger guard...a position which has somewhat fallen out of favor in recent years, but I'm not about to change, due to long familiarity.
Shooting positions can affect point of impact. Offhand impact will likely be different (usually higher) than point of impact achieved by shooting off a bag rest, or field rest if the position of the support hand changes slightly or downward pressure is applied to the rest, that won't exist in an off hand position. This can be minimized by striving to just lightly rest the gun (even the barrel...carefully) on the support rest, and being conscious on the position and tension of both hands, to mimic your offhand position.
