Tesla recommends in the User’s Guide that you should plug your car in whenever you can, so its Battery Management System can keep the battery happy. This is the formal answer to the question, even if you don’t drive much. Put another way, A, B, C, Always Be Charging.
If you can’t plug in all the time (either due to no charging capability where you live or where you’re visiting), then charge when you can. It’s a good idea to charge if you’re approaching 20% state of charge, as that is when other features like Sentry Mode stop working.
There is no need to drain the battery and then charge it up to maintain health. Some have recommended draining the battery to below 10% and then charging to 100% to recalibrate Battery Management System so that the remaining charge percentage gets more accurately reset once every quarter or so, but there is no known direct evidence to support this. Doesn’t hurt to do it infrequently, though.