At HomePointe Property Management in Sacramento, we suggest every vacancy be rekeyed between every tenant. There are a couple of reasons for this.
The number one reason is to provide a secure home for your new tenant. There is no sure way to know if the past tenant kept a key, gave a copy to someone, or lost one. Even if you got back every key you originally gave them, who knows if there is a key out there that the past tenant made.
The bad news would be if there is an unforced breakin shortly after the new tenant moves in. If you are accused of not providing your due diligence, you can go a long way to get out of a claim by showing that the locks were changed between tenants.
So the second reason for rekeying is to protect yourself from a lawsuit!
Rekeying does not mean buying all new locks. You could keep some extra locks and swap them out each time a tenant vacates, or have your locksmith change the keys out by changing the pins.
Do not try to save $50-100.00. You do not want to be held partially responsible for a breakin.