I actually have a US and a Canadian address in my United profile so I can change the point of sale at will and yes it makes a huge difference on some flights but for POS Canada, the only difference used to be that I can book the lower fare and get all my United points (if I book on AC, I get 25% in the lowest class).
But the point is I'm doing today YYZ-YQM-YHZ for half the price or anything AC was offering me (and that route was not an option on AC.com).