When I was living in mine during weekdays, I would do as you during the 3 day weekends; water off and elect on. I also left the gas on for 2yrs straight because I used it to cook, but the fridge could also fall back on that if needed (the power would die randomly for up to an hour at times where I was during the summer).
I left the A/C off. I didn't see much point to running it for 3 days while I wasn't there (and having to pay the elect for it as well). It took about 30min to get comfortable temperatures when I arrived and turned it on. The only down side to not having it on was that the mattress was heat soaked, and since I arrived at the end of the day Sunday, the bed was still warm even though the interior temperature was fine. Made it a little hard to fall asleep for me.