Any non-Federal Reserve Note, like a US Note or Silver Certificate, should be kept out of circulation, if not for any other reason than banks are in theory required to return them to the Treasury for destruction. By spending them, you may be temporarily putting them into circulation, but as soon as they reach a bank, they will either be removed by another collector, or sent to the Treasury. For reference's sake, I have seen one National Currency note in 'circulation', although as I mentioned in another post, the bill was ambiguously the property of a teller, while physically still in the till. This is why I don't understand why Where's George offers tracking for US Notes (most famously the '66 $100 US Note, which is a rarity.) By ruining those bills with a Where's George stamp or marking, it increases the chance that they will be spent or cashed in at a bank, and returned. It also reduces overall collector supply.