You'll find many books giving different figures, but for SW
most fall in the range of 1" per 3 gallons to 1" per 5 gallons. That assumes full grown fish size... not the size they are right now. I think that's a fair rule to do some early planning for your stock list, just so you don't think you're going to put a ton of fish in there. But if you're going to use the "rule", you have to take into account the things that have already been mentioned. "Minimum tank size" is really important, as well as what type of eater/pooper the fish is. Some 4" inch are waaaay messier than other 4" fish, and should be stocked accordingly.
While I agree that any "rule" shouldn't be followed blindly, I still think they have their place in the planning phase of things.