Living about 30 minutes south of YYB, I'm forced to do this quite often actually. It's usually pretty evident the night before if flights will be delayed or cancelled, if I see that is the case, I'll usually just be proactive and drive up to YYZ and catch my connecting flight. I've never called ahead and I've never had a problem. When it comes to the unused portion of my ticket, if it turns out to be cancelled or delayed for long enough where it would've affected my connection, AC has always refunded a portion after the fact when I call in. Obviously I don't bother if I drove for nothing and all would've worked out.