Border - No Problems, been back and forth many times. The most I've been asked is where and for how long before I go back home.
BC Ferries - The Hazardous material form is easy. There is no charge, and you get to load first.
No matter what you have in your tanks, it's air. Just air. Nothing but air. (Don't ask questions, this comes from the guy that trains the safety people at the docks)
If you can get the form ahead of time, then fill it out and all you have to do is give it to them - and make sure you arrive a few minutes early, you sometimes have to take it to a particular place other than the ticket booth area. (They're all very good at telling you what you need to do.
If you try to hide your tanks and someone notices, you can be denied boarding - I'd rather not take that chance.
I did have a small annoyance, once. The guy wanted to 'inspect' our tanks ... for some reason he thought it would be safer for us not to have the first stage on our stage bottles - we were confused how that could possibly make them less safe, but disconnected them and we were off.