"I'm curious if anyone here selects their rims for stiffness, and if so are steelies any less stiff than alloys? "

Its a good not yet well answered question, IMO. We seem to bend over backwards to reduce deflection elsewhere, when there is almost zero data to justify or quantify the problem elsewhere in our suspensions, my favorites are 11/16" TR and LCA bottom gusset plates. At a Ron Sutton track toy day long seminar in Ohio last year I attended, there was at one point he a panel of experts/vendors spoke about various pertinent suspension items, that included a wheel manufacturer. They stated wheel deflection is real and an issue, but not in their wheels. I asked them, and this point dovetails with another pissing match thread on this forum, how does brake induced heat effect the stiffness of their alum wheels, since at 200-300F alum loses a considerable amount of its strength compared to steel, and would say a heat barrier coating be advised to retain this stiffness they first mentioned?. They scratched their heads. Bottom line, if an alum wheel is flexing, it will be only a matter of time before it cracks. I do believe however elevated temps, lessons that propensity to crack.

Edit, if the amount of bending does not increase, but if the load is the same, the strength will be less, the bending amount will increase, and cracking issues return.