Pipe Threading Kit

Pipe Threading Kit
Pipe Threading Kit 2 Pipe Threading Kit

How to install shower mixing valve that has threaded ports?

I purchased an American Standard "Cadet" tub faucet/showerhead kit from Home Depot. The mixing valve has threaded female ports. Do I just screw in a threaded male copper adapter (with Teflon tape on the threads of course), then solder pipe to the adapter? It seems awkward to have threaded ports on the mixing valve because a) once you solder the pipes in place, you can't tighten that connection anymore anyway; b) it seems risky to have a non-soldered connection here, since it will be enclosed in wall/tile and I won't have an access path if it leaks in the future (I have back-to-back bathrooms). Or can I put solder in the threads?

You should be able to make all your connections and TESTING FOR LEAKS (you may wish to test for 24 hours to assure all the air is out of the line) before closing up the wall. Unions are not necessary as they are more prone to fail over time than the sweated and screw in fittings. Screw in and tighten the male sweat fitting into the v alve.
Then, sweat the rest.
Avoid getting the valve real hot so as not to damage the seals.