If the valve cover is as far as you're going, you can skip disconnecting the battery. If you're going to remove the alternator, then disconnect the battery. If you're doing ANYTHING where it's possible you'll put a wrench across a live terminal, then disconnect it.

Re-using the valve cover gasket is maybe 50/50 whether it'll leak. Even then, it'll be a slow-ish leak. IMHO, the valve cover is pretty easy to remove. If the gasket leaks, then get a new one & put it on. It's not a big job to take the VC back off again. If you're working on someone else's car, or if you don't have 1/2 hour some evening to repeat the job, then get a new one to be sure.