I've been going through nickels like a mofo lately trying to complete my 1938-2008 jefferson collection.
As with all coins, in TIME they DO go up in value. The questions then are: how MUCH time? and what is the MINTAGE of the coin? Lower the mintage and the longer you hold onto it the more it will be worth down the line.
I've been putting the sub 100mil mintages/pre 60 stuff into a bag as I find it while trying to complete my nickel book... I have yet to decide what out of that bag if anything to keep in the long run.
Naturally war nickels are keepers... as are the like 3-9mil mintages coins. But whether or not to keep those 20mil or 50mil or 90mil mintages is a matter of personal opinion which would include how many of them you have... how much space you have to store them, and how long you are willing to store them.
~Dave