These days, it's pretty customary for couples to want money in lieu of a wedding gift. Whether it be for their bridal shower or actual wedding, we're always being asked if there's a polite way to share these desires with their guests.
While we wouldn't recommend adding this type of request on a wedding invitation (after all, you don't want to imply that guests HAVE to give you something) we do think there are other creative, more subtle ways to get the message out. We find using your wedding party and close family members as to help spread the message works best. In our experience, many times guests will ask if the couple is registered anywhere or if there is anything specific they'd like. This is the perfect opportunity for your close knit group to share that you'd really like money in lieu of a gift.
Having said that, sometimes it's still a good idea to register or have a few stores in mind that you wouldn't mind receiving gift cards from. Some guests still may not want to just give money as a gift, and would appreciate knowing which stores you frequent so they could at least give a gift card.