What they're likely referring to is that if you shoot lots of .38Spl in your .357 mag and then don't clean the cylinders before shooting the magnums... the cases can get stuck b/c of the carbon build-up from the shorter .38Spl round. As far as the ballistics involved, there is zero reason why you can't or shouldn't shoot .38Spl in a .357 mag weapon. Some may be worried about the increased distance the bullet will have to travel before hitting the forcing cone, that perhaps the bullet will bounce out of the cylinder and then hit the cone off-center causing copper or lead shavings... I've never worried about this when using a gun with proper timing and you shouldn't either. So, unless your friends clarify exactly why you shouldn't shoot .38Spls in your magnums as often as you want, as well as demonstrating why your gun will create the issues discussed (apart from the carbon-fouling thing), ignore their BS. As stated, you do need to clean your cylinders after shooting the specials if you plan to shoot magnums thereafter.
Happy shooting!!!