quote old_salt: "The reservation you make is good for duration of trip. "
Correct. But I'll add that if you enter the park without clearing Canadian Customs (e.g., at a remote entry location like Prairie Portage), you'd best be in possession of a RABC (Remote Access Border Crossing) permit.
Otherwise, the only reason you might hit a ranger station on the way out is to pay for extra camping night, if for some reason your stay was extended beyond the intended period.
dd