I have also never heard of this "rule". I can see where you may want catfish to balance out an aquarium by having activity at the bottom as well as at the top and middle of the water column but having catfish, I would suggest, is by no means a necessity. The way it seems to come across from your LFS is that catfish are likely to provide some mission critical function (provided you have enough) and this is, in my opinion, not true. Catfish will look for food (i.e. what *they* find tasty) which will probably help reduce the amount of uneaten food in your tank. It is unlikely that they will catch everything, and even if they did, you would still have to carry out regular maintenance in order to get rid of waste products introduced into the water by your fish (including the catfish).
Your best bet (if you want catfish) is to research the ones that you find interesting and buy an appropriate number based on the fish-load your tank is able to handle, the general compatability of the catfish and wether the type of catfish you want prefers a solitary existence or the presence of others of its own kind.