Does it make sense to use a nipple to connect a tub spout to copper?
Typically you would use a copper sweat by FIP 90° elbow (often called a wing back elbow, as it has tabs on it to allow for securing to backing). Then screw in a test nipple (or use a nipple and cap) while the tile is being done. Once tile is done, use a brass nipple of the correct length (best to buy a few of different lengths and just return the ones that were the wrong length).