Even if I had the money, i just wouldn't feel right leaving the heating or aircon on. We have it, but very rarely use it because that thing eats up electricity at a very alarming rate in this country! We resort to using fans instead, on and off through out the day. It works just fine.
I'm used to switching appliances off when I am not in the house. The only time I switch the air conditioning on is in the bedroom to cool it down before I go to sleep and then switch it off before I do sleep. It's about a balance of comfort, cost and saving energy. If you do have animals and there is no ventilation it would be a different matter. I don't find air conditionign good for you, but you do need it in some countries.
I don't think it's a good idea to leave it on. There's a limit when it comes to using such things that we need to think of. There are things to think about like, electricity consumption or whatever energy you're using, unless it's powered by the sun and don't have to pay the bills and isn't wasting any man made energy that can run out.
