[Users] When does an AVP get destroyed? What if you delete an non-existent AVP?

Does the AVP get deleted at the end of a transaction? At the end of a specific dialog? If the answer is "transaction" (and I think it is), what exactly constitutes a transaction? e.g. when an INVITE is received,is the transaction completed when the corresponding "200 OK" is processed (or CANCEL)? Also, if I have 'avp_delete("$destination")', and the AVP '$destination' has never been set, is this "bad"? or a no-op? The above two are related. I am using AVPs to move information around in my script, and am wondering how much cleanup is necessary (i.e., delete every avp i populate, etc., etc.) cheers