There is something to it, but not a tremendous amount. The total volume of the backing you need for the fish you are targeting plus the volume of the fly line must fit within the spool so that when completely reeled up it does not bind with the cage.
Also, the weight of the reel should not be so light that the rod tip feels heavy or so heavy that the tip feels too light.
Other than that, the numbers are guidelines to reduce the guesswork for us consumers.
One additional thing is aesthetics. Yes we all tend to care a little about that as we should. If the reel satisfies our own opinion of proper fit (diameter of the reel seeming right) then it's a good fit for you.
Many great reels coming out in 2006! Dont' forget to check out our Flyfishing Retailers Show report which should be up in a couple of days (we're at the show in Denver now).
IFFF Certified THCI @ 2005
Capeflyfisher Guide Service
Island Hopper, Guitarist, Incurable Dreamer
and Founder, Worldwide Flyfishing Forum