If weight was the ONLY concern, consider that a full bladder weighs about that... pee before you ride, and you get the same benefit.
However, the *quality* difference between good wheels and average wheels is miles beyond simple weight differences.
My steel fast-bike weighs 21 1/2 lbs. I'm 60 lbs overweight right now, and I honestly don't think anything I do to the bike would have any effect compared to losing some of my personal butt and belly lard. But if someone were to offer me some hand-built wheels, I'd jump for them!
Fine wheels are a joy forever... 
Caveat Emptor: I'm comparing a road bike to a mtb, but I still swear that fine wheels are like fine wine whether 26" or 700c.
Last edited by KnottedYet; 04-21-2011 at 07:38 PM.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson