I can only speak as a Canadian, but I have always been scrutinized a lot more by the US boarder guards than the Canadian ones. I have yet to cross over to the US for the purposes of an IBA ride, but any time I was just out for a ride and didn't have a specific destination in mind they didn't give me any trouble. Same coming home. They are mostly concerned about whether or not I have anything to declare. Again, I don't always have a destination planned on re-entry, just ride until I see a motel I want to stop at and haven't had any trouble. I wouldn't shy away from telling them you are attempting to complete an IBA ride, but I wouldn't say anything that makes it sound like you are in a race of any kind.