No need to install unions. You can cut your lengths of copper and solder them up with your male adapters before installing them on the valve. Depending if there are any studs in the way or not you can even solder the 90* elbows and the lengths of copper for your risers and then assemble them to the valve before installating the valve. Then just make your final solder joints far away from the valve.
If you already have your supply risers in place and just need to connect to the valve, use as long of piece of copper as you can to solder your male adapters on to. Then install them into the valve, get some Heat Sink, or wrap with wet rags and make your final solder connection there at the 90* elbows. You dont want to burn out all your pipe joint compound. Do the same for the shower riser, solder up the male adapters to the copper and then thread it onto your shower valve and volume control or diverter.