George,
I assumed your suggestion was standard practice. When I was taught how to sight in a rifle, the one shot sighting in with a bag, or a rest as you describe is exactly what I was taught. If the backyard boresighting gets you on paper with a crosshair target, and you have a decent scope, it should not take more than two or three scope adjustment shots to be shooting off the bag within the accuracy of the rifle.
Because I am obsessive about accuracy and physically disabled, I must shoot with a bag or a bipod. I recently mounted a Harris Bipod to the Ruger No. 1 220 Swift. I find it is more stable than shooting off a bag so I plan to get one for each of my rifles.
