You are probably fine with the older wallet.dat backup for the most part, but it is better to have the latest good backup for a few reasons.  Your wallet.dat contains more than your private and public keys aka addresses it also keeps your personal labels you have given to them as well as your change addresses.  Your Pandabank starts with 100 change addresses, it uses them to receive the change from your transactions, when needed new change addresses are also generated and added to your active wallet.dat.  Change addresses added could be missing in an older backup of your wallet.dat and although they are yours using the old backup potentially would not have them, leaving you with a smaller balance and the possibility of losing those change address PND.  If you added new addresses yourself or labels since the last backup they could be missing after restoring a PandaBank from an old backup.