The risk, I think, is in negotiating the on-us check in cases where you are not in a position to know that the individual presenting the check is authorized to negotiate it on behalf of the payee. In other words, you could have a fraudulent (forged) endorsement, and the cashier's check could then be fraudulently used.
Admittedly, if you are dealing with known payees, the risk is mitigated.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8