I presume you're asking with regard to a Border to Border ride. If so, the rules note:
"Please note: You must prove you crossed the border with your motorcycle to both Mexico and Canada. Due to the language difficulties, we are flexible on documentation but still, the burden to prove you made the crossing is on you. Some acceptable forms of proof include: - Written statements from Customs agents that you crossed the border (use witness form). Be warned that these maybe difficult to obtain. - You may cross the border and obtain a time/dated receipt for a purchase and then cross back into the USA to obtain the ending witnesses. Pictures are also considered evidence on this ride that your motorcycle was in Mexico and Canada."
So the question is: does a passport stamp prove you and your motorcycle crossed the border? I think not. I would therefore recommend getting a DBR or taking a photo after crossing.
Ira Agins
Iron Butt Association