joehardy,
While entering the US, I show my passport where I have my I797 and I94 stapled on page 1 next to my photograph. While entering Canada, I just show them the Maple Card.
I have (except once to declare product samples) never been asked to go to the secondary inspection. I have always explained my purpose of entry while inside the car. The US fellas do not and have not made any entries in my passport or anywhere else other than my licence plate number.
For the most part, the US guys ask for my citizenship and I tell them along with the purpose of entry and they let me in. Canadian guys used to ask my citizenship when I used my landing paper to enter. Now with me sticking out the Maple card in front of them, they do not ask for my citizenship or at least they haven't since I got the card. Hope this helps.