As it turns out, there are only a few key codes commonly used by the banks for homes being foreclosed.  If you know where to look, you can easily find out what these codes are.  So, any would-be thief likely already knows what those codes are and has a set of keys already made up so that he can easily gain access to a bank-owned property.  Usually one can easily identify homes that have been re-keyed and simply try a few keys until the right one is found.  I have often opened homes that have previously been re-keyed using such a set of keys that I have made up for this purpose.  I can usually get the right key the first or second try!!  How secure is that?  ...not very.  So, unless the bank requires you to use a specific code, I recommend using a unique key code.  It can be one you come up with on your own or your locksmith should be able to provide one for you.  Let's face it, if someone really wants to get into a house, he can.  But let's make it as difficult for him as we possibly can. 
     And if you are a new home owner, I recommend that you have your locks changed as soon as possible after you move in; because there are a lot of folks out there who have a key to your home, and not all of them are honest.