Is it Tacky to Write: "No Boxed Gifts" on a Wedding Invitation?

My sister is getting married this August & she is the process of creating her wedding invitations. Last time I spoke to her she mentioned to me that her & her fiance were thinking of putting "No Boxed Gifts" at the bottom of the invitation. She asked me for my opinion and I told her how distasteful I thought it was, however, her fiance disagreed and said that most of the time when people buy you gifts as a wedding present it's usually something that will never be put to good use & that cash gifts are better because they can help pay for a honeymoon and wedding bills. They both got out of school not too long ago and have a tremendous amount of school loans on their shoulders but either way this seems extremely tacky to me. What do you guys think?