I think there's room for flexibility - when I first became a 'non-resident' I wasn't sure that I wasn't still a resident (came back to visit when I'd only been out of the country a short while) so I filled out the declaration card as a resident and used the associated line. When I got to the front I was gently scolded ('you'll always be a Canadian - but you're not a resident anymore...'), and told that next time through I should declare as a non-resident but use the same line.