A timely question since there is presently a heat wave where I am :-)
I have followed a couple of strategies on this over the years.
As a general rule, and lacking any better reason, I turn the AC off if I am going to be out of the house for more than a quick trip. It won't take that long to cool off when I get home.
For the bathroom and some other areas where built up humidity can become a problem, I either run an exhaust fan or a small dehumidifier. In my case, the dehumidifier will consume less energy than the AC (local specialized solution vs generalized solution).
For larger areas, I run a fan to keep the air circulating, again rather than run the AC. One of my newer AC units has a very energy efficient fan mode, so on that unit, I use it.
To give you an idea of my environment, this week the temperature has been pretty consistently at or above 30 degrees C with humidity adding 10 to that.
In the case of central air (which I don't have), I believe some have programmable thermostats which can keep the environment "cooler", ie 25 deg C, when you're out ... this seems reasonable to me.
As was mentioned here if there are pets to be taken into consideration, or any other reason (such as it gets absurdly hot where you are and food in the cupboards may melt) then running it at a low rate is probably the best answer.
I also make liberal use of drawing shades on my windows and doors to reduce the effect of the sun beating in and warming. This really has a huge impact here.
Another thing I didn't see mentioned was that if you are going away and have high daytime temperatures but moderate relative nighttime temps, you can combine turning your AC down with an electrical timer, so that the AC is turned off overnight. Since you're not going to be home, comfort isn't the main issue. A temperature controlled thermostat like I mentioned on the central air or modern AC units is the best bet IMHO, and I'd set it on a higher-than-normal-but-still-cool range (here that would be 25-27 C, when I'm home I prefer being around 22 C).
Hope this opinion helps.