Physics and experience tell me differently. If the stays are made out of the same gauge material and are the same shape and profile please explain how one would move 4mm and the other wouldn't move at all.
(meant to quote well biked)
I'm not sure they are the same shape. Drive-side chainstays usually have a considerable dimple to clear the chainring, in addition to the dimples both chainstays usually have to clear the tire. I'd expect the difference in bend (if any) to be concentrated where the chainring dimple is. Certainly at that location the drive-side member is less resistant to sideways bending moment than is the non-drive side member.
And what makes sure the owner's applied forces are symmetrical?
I've had several frames gone over by a good aligner. In all cases the riding improved significantly. I'd let the frame work be done by an experienced person who understands it and is equipped.