I've crossed the border regularly at Detroit, Port Huron and at the Niagara/Buffalo crossings over the years and have never had a problem with a cello visibly in the car. I've always had either a bill or insurance appraisals in my name, but I've never had to show any documentation. I just avoid taking anything with ivory or other materials from protected species over the border to be safe and to avoid all the hassles with the CITES certification and port requirements. All of the horror stories I've personally heard over the years were ivory related.