Inch per gallon is really only a guideline, for fish under 5 inches, and not even all of them. I just research what the particular fish need and do a best-guess as to whether it will go in my tank with the other fish.
I pretty much do the same as Teelie. But the inch-per-gallon is pretty useful for beginners, for the most part. To me, it depends on the type of fish (including their adult size, disposition and needs), the amount of work I want to do (whether I want to stock it a little light so I don't have to fret so much over the water changes, etc) and what the average advice is from all the fish folks I know.
. I've heard so many, but inch per gallon seems quite popular.
