How often should one empty hot water tank

Actually you are draining the water heater when the low point drains are open.  Just open the hot and cold water faucets at the sink to allow it to vent.  But, it will take a long time to finish so removing the anode rode is faster, if time matters.  

Those low point drains are for the plumbing and not directly draining from the hot water heater. Make sure your water heater is switched off. I take a 1-1\16" (I believe that's the size) socket and unscrew the anode. Be careful in that it may have some pressure behind it. As the water is draining lift up on the water relief valve to maximize drainage. I carry a roll of the white sealer tape with me but I find that I don't have to replace very often. What you want to be careful on is screwing the anode back in. Make sure you start correctly so you don't strip the threading. It takes a little practice a few times to get it back in because you have a tight space to get your fingers around to start threading it and the weight of the rod once you slide it in wants to pull it down. Just make sure to hold it up level once you slide it back in and then find your starting thread point. It's really a very simple procedure. One last note don't overtighten the anode. If you have tape on then you just need a snug fit that doesn't allow water to leak.

Matt,

Thanks, John!

Thanks for all your responses!  I really appreciate it.  I'll have to look into "blow[ing] out the lines."  I certainly don't want mold or other unpleasantries in the lines.  Thanks again for all your input, everyone!