That fact is that when you're rested you're not shooting the same way as freehand, and that's not the same as using two hands standing. If you really want to be on, I'd suggest scoping in (or sighting in) for whatever you shoot most.
I scope my rifle in for how I shoot, my sweet spot is about at the center of my finger (from knuckle to nail), it feels good there on my romanian. I scope it in rested, then if I notice I'm shooting left, I ajust for that, as my main shooting is from a standing position, shooting critters.
It's a cunundrum, but hey, get 3 guns, sight each in for whatever you'll use it for and you're good!
You can use a different spot for each position, as the gun will kick back a different way with each grip you take, but it's probably better to get used to one method of shooting and stick with it.
"The distance between insanity and genius is measured only by success."