Laura, I live in Canada and I say 'yeah' constantly! I can't imagine starting to say 'yes' all the time because it seems a bit unnatutal to me - just like I say 'can't' instead of 'cannot', and 'I'm' instead of 'I am'. Its basically the same deal for me with 'yeah' only its closer to being a slang abbreviation rather than a grammatical abbreviation.

Mulder: Imagine if you could come back and take out five people who had caused you to suffer. Who would they be?
Scully: I only get five?
Mulder: I remembered your birthday this year, didn't I, Scully?

(The X-Files)