Leave an Access Panel
If you can, do what the architects of old did: Install an access panel on the opposite side of the wall behind the tub or shower faucet.
This was common years ago, and access panels can still be found inside closets of many an older home. The access panel should extend one foot above the top of the faucet and all the way to the floor. It will prove valuable if you have to replace the faucet or work on the drain trap at the base of the tub.
Be sure to remove the cartridge from the faucet to flush the water lines. Small balls of solder and other debris can clog the ports inside the cartridges. If this happens, you may end up buying a new cartridge before you take your first bath. It takes only a few moments to remove the cartridge. Buy a faucet that has built-in shut-off controls so you can flush the lines by yourself. If you don't have shut-off controls, you will need a helper to operate the main water valve while you stand in the tub with a bucket to catch the debris-filled water.
-- Tim Carter