It is still best to do both.
But, I'd rather see you close them and not bother setting them to nothing than vice versa. In theory, every procedure cleans up after itself when it completes. This was less true in practice in earlier versions. Closing the recordset seems like to me it would be releasing the file lock whereas setting it to nothing has to do with the memory on your machine. If you have to reboot a client because of memory issues that is less of a concern for lost productivity while you wait for you file locks to get killed.