Is it necessary to drain the hot water tank before changing the shower stems? My hot water tank is in my attic.

No, it is not. First, close main cold water supply to your house. Open h/c shower stems in the shower you want to repair. Water will drain out. When no water is coming out, than it is safe to remove stems.

Some faucets come with shut-off valves build on to incoming hot and cold water side of the faucet. You remove trim and see if your faucet has those. If positive, than shut them off at the faucet. In this case, closing water off for the whole house would not be necessary.

You may also shut the hot water off at the water heater. This will allow you to replace the hot water stem and washer assembly and still keep your house pressured up. You may then follow Milos advice. Good luck, Tom