Came through YYZ yesterday traveling YOW to SFO. Had a tense connection, made worse by deicing in Ottawa.

Arrived, followed transfer signs, argued our way in (showing our boarding passes to the person at the door wasn't enough, she wanted us to go get boarding passes and we had to argue that was what we had shown her), then waited for our bags. They arrived and we go to line up for immigration,

To leave the baggage area, you must show your baggage tags and have them matched to the bags you are taking into US customs and immigration.

Of course, these were attached to the boarding pass for the previous flight, now buried in some forgotten place since we've never needed them in the past. Which made the process slow since no-one else was expecting this either.

Have no idea whether the check is required on the way back or not.

BTW: one of the two bags didn't make it from the US luggage belt onto the plane. It did make the next flight, 7 hours later but then took 14 hours to make it from SFO to our hotel. Despite E status, marked priority, numerous phone calls and the fact the file pointed out there was temperature sensitive medicine in the bag.