May I recommend that you use CPVC plastic pipe for the shower riser? That way you can cut the length of pipe, put a shower ell on the top end (which glues on and has female threads for the shower arm), screw an adapter into the valve, stick the pipe up, glue it into the adapter, go around and screw the shower arm into the ell. The plastic pipe is more flexible to thread through the wall and cemented joints are much easier than soldered joints in copper.
Copper and plastic pipe have been in use long enough to make comparisons between the two now. We have found that water going through a copper ell will eventually erode the copper and cause a leak (the water is trying to stay in a straight line of travel puts more pressure on the back of the ell, plus the abrasive minerals in the water). Meanwhile, CPVC is taking the wear very well and doesn't have any rust or corrosion to battle, also the cement does not burn my hands when it drips on them.