I certainly would not drain it every time, but occasionally the water heater will develop a bad smell, depending on the water you run into. Draining and refilling will take care of it, when that happens.
We live full time in our motorhome and I only drain our water heater when servicing it. When we used it part time, I only drained the water heater if it was going to sit for more than a couple of weeks. Most seasons it was drained only occasionally, and in the fall when we would winterize it. I did that for about 25 years of RV use before we went on the road.
If you get an odor of rotten eggs from the hot water it means that you have algae growing in it. There is a variety of algae that will live in water heaters which causes that. If that should happen you need to sanitize the system to get rid of it. Draining the water heater might correct the problem if you leave it dry for long enough, but sanitizing is much better. You can find directions here: http://www.adventure.1tree.net/index.php?option=com_content&view=article&id=92&Itemid=106