I like to rest muzzel on bags when working up loads for acuracy but hand on the bags when sighting in for offhand shooting. I myself can be more consistant with muzzel on the bags and consistancy is what its all about when working up your best loads as has already been said.
Spot-on- you will usually find with different guns, and/or perhaps different loads, the rifle will shoot best with a different or even slightly different location of the gun on the bags. This, of course, is due to the harmonics of the load in THAT rifle, rested in THAT manner. Again, it's all about consistency when shooting ML's (and other guns).
My .40, for instance, when prone chunk shooting, prefers to rest the 1st. entry pipe on the bags - not the muzzle or 2nd pipe nor the entry pipe - but the 1st pipe is it's 'sweet spot' location.
Shooting off bags and letting her bounce, not forend held, it prefers to rest on the entry pipe.
One MUST shoot the rifle enough to get to know it. If possessing a lot of guns, I suspect it would be most difficult to remember each gun's idiosyncrasies - without a diary for each gun. This was something taught in the CF rifle, competition shooting arena and is a good idea with our ML's as well.