I also agree with sixrivers. I will say though, that imo it isn't so much the size of the fish but the situation and the temperment of the fish that would be the big question mark. 10lb fish that is a bit more on the lethargic side, in softer water, probably no problem with someone who knows how and when to pull on the fish. At the same time, a 4-6lb fish right in from the salt and in heavy current could prove to be way too much for 4/5wt imo, never mind a 12-15lb fish.
Lots of factors to consider, but in order to keep it simple it makes much more sense to have a rod that is sized to handle a fairly broad range of conditions. Always best to err on the stout side if in doubt; I'd hate to have to break off the few really hot fish that I do manage to hook, and I would NEVER want to have to play them to death just because my rod was a bit too light.