Do you always recommend that your clients change the locks on the home they just purchased? If you don't, you should. This is especially true if the home was a foreclosure. Foreclosures are generally rekeyed to a "master" keycode that any number of agents, vendors, and other contractors, etc. have access to. I personally have over 25 different master keys; this should give you an idea of the level of accessibility of a foreclosure property.
Not only that, but many buyer's agents have been known to give out the lockbox code to their investors (a real no - no), and WHO KNOWS what happens after that??? Did the investor make a copy? Two, so they can give one to their partner for convenience? It happens. Fannie Mae makes it a part of their addendum to the contract that the buyer has to have the home rekeyed right before closing. (at the buyer's expense, of course). Frankly, I think this is a wise policy that all lenders should embrace.
Even if the home was not a foreclosure, it is still a good idea to rekey the home. There is NO DOWNSIDE TO REKEYING A NEWLY ACQUIRED HOME! After all, the previous owners could have given keys to anyone -- and probably did -- (neighbors, friends, house-sitters, etc). There could also be a key hidden outside somewhere that the previous owner forgot about...you know, in one of those fake rocks in the garden...The possibilities are endless. At the end of the day, there is really no reason NOT to rekey a newly acquired home.
Another option is to replace the locks. However this option is more expensive and because you are replacing locks, you aren't recycling the old ones...If possible, I always opt to recycle...
Bottom line, rekeying is one expense of buying a new home that is well worth the money and one that should never be overlooked. Please remind your buyers at closing to do this before they move anything into the home. Failure to do so, as REALTORS, is nothing shy of irresponsible to our clients.