I never shut mine down when on vacation because I had to do it the breaker and the breaker box is behind a library shelf and books have to be taken out, etc. My new water heater has a shut-off at the water heater and a digital control with vacation mode so I put into vacation mode for two weeks. When I returned I set it back to normal operation and the next morning the hot water stank to high heaven of sulphur/rotten eggs. I then looked around and called a plumber friend who said that during the cooling process the water will be at a temperature that is optimum for bacteria growth for quite a long time and that sulphur bacteria has probably grown within the heater. He said that if the smell doesn't go away after 2 weeks, I will have to disinfect the water heater with bleach or hydrogen peroxide. So no, don't turn off your heater while on vacation.