But the commercial shop I take mine to blasts them with walnut shells.

Metallurgically speaking, blasting with sand 'pits' the soft aluminum, and worse, pits it with sharp little craters/cracks. It seems to me this would generate millions of potential stress risers. Shot peening, OTOH, is done with larger media, like tiny spherical ball bearings or even lead shot...the idea being to induce compressive stresses through cold working into the surface of the metal. With these residual stresses impressed (pun intended) upon the metal surface, fatigue crack initiation is retarded.

I dunno if it's worth it on the alloy on your wheels.