Canada wasn't a stereotypical nice country back in the 1950s. You are right about the border and most of the population living close to it. But we have large cities in the north. The provincial and terriotrial capital cities would have been targeted. Not to mention all the northern military bases and early warning radar stations set up under NORAD in the arctic.
Canada in the Fallout Universe might have even been a nuclear power. From my understanding Canada rejected nuclear weapons in the 1970s. America wanted to place nuclear weapons in our arctic territory. The reason behind saying no was two fold. Canada was becoming gripped by Liberals and the idea of nukes in the Canadian arctic was pissing off the USSR.
So it could be possible that Canada in the Fallout Universe accepted nuclear weapons and placed them in the North. Which would have made Canada a nuclear power in the Fallout Universe.
Either way thow, America would have put nukes in the North once they started taking over in 2066, and since Canada became part of the USA in 2077. It would be targeted just like any other part of the United States by countries all over the world.