As stated above....Fire away with all the .38spcl ammo you want. Be sure to clean the cylinders though because the .38spcl round has a slightly shorter overall length than the .357mag and over time (without cleaning) you will develop a powder residue ring that may inhibit the .357mag from sliding all the way into the cylinder.
You would have to pretty much absolutely neglect the gun for that to happen but I've seen some pretty gnarly guns in my day where I don't think they were cleaned even once.
Keep it clean....shoot 38spcl +P38spcl or .357mag all you want.