I am actually in the similar situation as you. I was confirmed funding with my “dream program” shortly after sending in applications to backup programs. When I receive interview weekend invitiations, I just politely decline saying something like, “I have already confirmed my acceptance with another program, and don’t feel right wasting the time of your department.” And thank them for the invitation obviously. I don’t think it’s a negative either way (going to interview even though you have no intention vs. rescinding application)