Every time in my life that I have flown out of Canada to the US I am checked at customs BEFORE boarding the airplane, and US customs is not Air Canada. I check in at Air Canada and THEN proceed through customs before going to my gate. My parents just did it, coming here, and they showed Air canada nothing, they only showed ID when they went through customs before leaving Canada as well.
I assume you will be going through it AFTER landing then? You need to make sure when you go through Customs, because if you go through it before boarding, then it's US Customs you deal with and not air Canada. Once through customs, they can't tell you to not board if you have photo ID, and if they do, it's easily argued since you've already gone through customs who are the ones that have the say whether you can enter the US or not.
I guess it all depends on the airports you fly in and out of.