I understand your concern about not wanting to beat up your gun . I have had quite a few K frame 357's so I can speak with experience . First off , don't let the " nay " sayers convince you that it is weak and should only shoot full magnum loads , occasionally . It was designed for the 158 gr , or heavier bullet .
I agree fully with what Cowboys says.I've also owned a few .357K frames and never had a problem with any of them.But I've always shot 145gr or heavier(lead) bullets in them.
When the word about the 125gr eating away the top strap went around,it spread pretty fast;much faster than the fact that after a certain depth,the ''eating away'' stopped.Bad news will always outpace the good ones!
BTW,nice gun you got there.There's a lot of guns that I've sold that I shouldn't have but amongst all of them,my model 66 is always the first that comes to my mind.Have Magnum fun with it.
Qc