The 64 is +P rated, as are most Model 10's. It depends on when they were made. Most Model 10's that are actualy marked "10" should be able to handle +P's; certainly any that are marked 10-5 or higher.
The model 64 is essentially a stainless model 10.
I would not suggest putting A LOT of +P's through a K frame .38, as it will increase wear and tear on the gun; however, practice with standard .38's with occasional +P firing should be fine.