This is the best analysis of the issue I've ever found:
http://www.gunblast.com/Butch_MagnumLoads.htm
Personally, I stick to either .38 Special ammo or .357 Magnum ammo with bullets of 140gr or heavier in my K-Frame (Model 66-2) and reserve the lighweight magnums for my N-Frame (Model 28-2). To me, it's not really that big a loss as I actually prefer the 158gr loadings anyway due to their deeper penetration and reduced flash and blast as compared to 125gr and lighter loadings. My typical .357 Magnum self-defense loading is Remington Express 158gr SJHP and my normal practice load is a 158gr LSWC over 14gr of Alliant #2400 with a Winchester small pistol primer.