In Theory I can copy the wallet.dat file from the Bitcoin QT program on my desktop to a thumb drive and if my desktop blows up I can get back into the wallet by re-downloading Bitcoin QT on another computer and placing the wallet.dat file from the thumb drive into the new program, correct? Could it be an old .dat file, or should it have been updated with each wallet transaction?
Bitcoin-qt generates addresses in batches of 100. A new address is used every time you request a new receiving address in the program and every time you send a transaction that has change.
So to be safe, you should create a new backup after every 100 (transactions sent + new receiving addresses created), or more frequently.
But with these considerations taken into account, then yes, you can use the backup file to completely restore your wallet if your computer fails.
