Assuming that you have sanitized your system before the season started, I see no reason to drain it between trips. We don't, and have never noticed a problem, such as odors, bad taste, etc.
Deane, Carolyn, and Kasey, our three year old yellow lab.
2011 Greyhawk 31DS,:) upgraded from a 2008 Greyhawk 25DB.
2003 Explorer Sport toad.