Please DO NOT put it on your invites! The ONLY acceptable way to request money is word of mouth. Tell your close family and friends that you would prefer cash, just in case anyone asks... If people ask you, tell them honestly, but still remember, you will get gifts that aren't cash and just to be gracious and thank the giver for the thought.
Oh my, please do not put on invites, or anything in the invitation that you prefer money. There is no polite way to come out and ask for cash. However, you can always opt to not register anywhere which will more often than not, lead your guests to give cash. A registery is a way of letting guests know what you need/want for your new life together. If you don't need/want anything then don't register and I'd bet you primarily get money.
