Winterizing & Summerizing for Snowbirds

If the water heater is turned off, should it be drained also? I've heard stuff can grow or sour in a tank of water over a period of time. Also wondering how all water lines in the house can be drained, or does a plumber need to be called to blow them out. We have a slab floor so no way to get under the floor. Would love to spend the winter away from northern Oklahoma but don't have anyone close that we'd trust to come in and check the house while we're gone, and winter temperatures can really vary from year to year. In the summer we always turn off the water at the meter, turn down the water heater, unplug everything except frig and freezer, and leave several light timers on, but don't have to worry about the house heater at that time when we're gone for a couple months.
Wagonmaster2