Basically what Bubba said.
I like to start out with mine just a touch snug. Not tight, but not with much daylight either, because I find that the longer I ride, the looser it gets. So if I start out with hardly any daylight, then by the end of the ride, it will have a couple inches gap. Sometimes more.
Mid-way on a several hour trail ride, if I get off to open a gate or something, I find I often want to take up my back cinch a notch.
Basically, I check it whenever I check my front cinch, because I frequently take up the front a notch or two also.