Think of it like this... the blockchain contains the data to determine a wallet's balance and the wallet.dat file contains the data to spend bitcoins, or rather, sign money away to other addresses.
If you're taking your money off-line, then yes, you should make several wallets and keep various denominations in those wallets. When you need to bring some money "online" you just open as many offline wallets as you need to get the amount you need.. that way, the rest of the off-line money stays safe offline. and actually, you don't really need whole wallets for keeping stuff offline.. you can use bitaddress.org to generate just the bitcoin addresses (and keys) (offline).
srsly? fucking A =D
btw, from everything ive read so far, it seems to me that bitcoins arent really stored on any file, but associated with a certain wallet which cannot be accessed without the wallet.dat file. the reason i think that are the instructions on how to make a 100% secure wallet. the way i understand it, the wallet.dat file you're supposed to put your money doesn't seem to be online at any point except for when you retrieve money from it. is this accurate or am i misunderstanding something?
also, wouldnt it be way safer to have a shit-ton of wallets with only small amounts in each (each with their own password of course) rather than having one wallet with everything in it? yes, it'd be tedious but safer, wouldnt it? (assuming you're able to memorize a high quantity of very complicated passwords that is)