The main reasin not to taper below appx 1/16 inch thick is the strengnth of the balsa. If you taper balsa to a knife-edge it may as well be tissue paper.

I have on some models laminated 1/64 plywood on the lower side of the control surfaces and then tapered the surfaces down to the plywood. That is for all practical purposes as close to the knife edge as you can get. It works fine. Its a bit heavy... so you get into cutting lightening holes in the control surfaces. The thin ply is fairly expensive. The performance gain is negligible if you are not trying to set a speed record.