Customs doesn't ask for it. You would only hand it to them if leaving US for good.
Airlines*usually* don't take it on flights to canada. but if they want it, they will usually insist until you give it, or until you end up calling for a supervisor. In the dozen times I flew to canada with an I-94 in my possession, it was never requested, and was always in my PP.
But I would have been up for the fight; you may want to avoid one.
So, if you prefer, take it out before you get to the airport, and staple it back in while in canada.