Just make sure that you have numerous $1 notes in your wallet and don't confuse them for $100 - US currency looks the same except for a couple of slightly coloured denominations.
Very wise advice. When in LA I was wheeled to the busstop by a porter, who was already being paid by the airline, I asked him to take me first somehere where I can change a $100 note for his tip. He told me that he has ample change. Indeed he had, but (surprise, surprise) nothing smaller than $10 bills. Thus he gave me $80 and after a long arguments gave me another $10. Prior to leaving the aircraft the stewardess suggested a maximum of $3 tip for this porter, but he was more crafty and made sure that he gets the better of a greenhorn like me. Btw. Americans never get confused when it comes to their notes or for that matter when it comes to money.