You'll lose some of the volume capability of the valve if you use pex. Often, they specifically say not to use it to interconnect pieces. Using it as a supply also means you won't get the maximum volume you could have. in a traditional setup, using 1/2" pex, with a divertor spout, it will back up into the shower. Since most stub spouts are 1/2", using pex to it would be a problem. I'd use copper after the valve for all connections. This will also mean that you can anchor it well.