I shoot almost all of my 7.62x39 from offhand position. Truthfully, my weaknesses are in the other positions. I would rather take an offhand position with support than an unsupported kneeling, sitting or prone position. I prefer to add support rather than modify my position as I find some aspects of the NRA positions very uncomfortable (especially sitting). I only shoot sitting from a Harris bipod.
As for more powerful calibers, I just can't afford to shoot as much .308 or 7mmRM, even though I reload.