Depends on what you are stocking......CM/L is ideal for most community fish IN/G is better for larger fish and some aggressive ones. If you are new to the hobby, IN/G is also safer, as a litely stocked tank is less like ly to have problems, in my experience.
Like DiscusLova said, this simply isn't true. It is a gross oversimplification.
Are you going to put a "larger" fish, like a 20 inch fish in a 20 gallon tank? It still works in the inch per gallon rule.
I copy and paste this out every time I respond in one of these threads. I've had it written in some form for the last 3 years, and it is holds very, very true:
Firstly, the amount of waste a fish will produce is primarily based upon the mass or volume of the fish. Consider that a 4 inch streamlined, torpedo-shaped rasbora will not be producing the same amount of waste as a rotund, beer-bellied 4 inch molly who is in a late-term pregnancy.
Secondly, the one inch per gallon rule becomes very silly in the limit of large fish. 20 one inch fish (like small tetras) can be very happy in a 20 gal tank, but obviously one 20 inch fish will not.
Thirdly, the species of fish enters into consideration. Fish from rapidly moving, highly oxygenated waters will require more water per fish than fish from slow moving stagnant ponds. This is where research becomes necessary. Also, some fish are very territorial. A 5 inch fish may claim much more than 5 gallons as its own personal space.
Finally, the fishkeeper's caretaking habits can change the balance as well. If you commit to increased water changes and caretaking, and you 'over-filter' your tank, you can get away with overstocking. But this is only recommended for the more experienced fishkeepers. Likewise, if you want to be lazier, you can plant your tank to help process the fishs' pollution but that requires an aquatic green thumb so-to-say. There are whole other issues for keeping aquatic plants alive.
Many, many factors that "one inch per gallon" cannot take into account. It is just a starting point, and a good place to help newer fishkeepers not run out and purchase as many fish as can fit in a small tank. To be completely accurate, all the above factors, and maybe several I have not even considered, need to be taken into account. Whatever the result will be, it will not be as catchy as "one inch per gallon" which again, is only a starting point.
So the point is, you have to research the fish you want to keep, and you have to know your own abilities.
I have about 60 inches of fish in a 29 gallon. But I do water changes and have a good filter. I haven't had a death in over 6 months. 
