This business of worrying about the long-term effects of firing .38 Specials in a .357 Magnum revolver is a total NON-ISSUE. After all, .357 Magnum caliber handguns have been around for what, more than 70 years? If, in all that time, there was really a problem, wouldn't it have become public knowledge by now?
Anyone with any sense of caring for their firearms will do regular cleaning, lubrication, and preserving of their guns. That is all that is needed after firing shorter rounds in a longer chamber, whether that is the .32 Spl/.32 Magnum, .38/.357, .44/.44 Mag, or whatever.
As for worries about chamber erosion from firing the shorter rounds in the longer chamber, that is utterly nonsense. Most everyone understands that flame cutting and erosion takes place at the B-C gap or the throat of a barrel, not in the chamber throat. The massive material in the cylinder provides a heatsink to prevent the surface heating and theoretical erosion that combustion flame produces. Look at military automatic weapons designs and you will see the point. Machine gun barrels and chambers are stellite-lined, chromium plated, or otherwise designed to resist heat and erosion at selected points: barrel breeches, chamber throats, B-C gaps, etc.
At one time, in the long distant past, corrosive primed .22 Shorts fired in a longer .22 Long Rifle chamber could cause problems, but that was from rust and neglect due to the corrosive priming. Modern ammunition is almost completely non-corrosive, except for some brands of surplus military ammo.
You can shoot .38 Specials as much as you like in in your magnum without worries.