Whenever you are carrying a significant amount of valuable items across the border, it pays to stop at the Canadian side of the border first, have them make out a (dangit, can't remember what it's called) note of ownership. This way, if you are questioned when you return to Canada, you can give them the note that states all the items that originated in Canada and thus is exempt from any tax/duty it might otherwise incur.
It can be hard to make the case that you were on a weekend trip to the US and that you took 30 razors and 20 hones with you for that weekend! The same thing goes for expensive jewellery, cameras, firearms (yes, I was hunting at the time and didn't want to leave my firearms in a chintzy motel room), ham radio gear and the list goes on. If you cannot prove that you had it with you when you left Canada, you could be charged taxes/duties as if you were importing them, when returning home.
Yes, a tourist would carry a camera... but not normally three $2000 cameras and 8 lenses. I've also have had issues with rather expensive ham radio gear, already mounted in the car, but the suspicion was that we drove to the US without the gear, purchased and installed it in the car while down South and then headed back north. Having that piece of paper filled out, can save you a bunch of headaches. When I have had them done, they want serial numbers if available and detailed descriptions so that if they check upon return, they can identify the items correctly. Going south with 1 old (junk) razor and returning with a $800 Custom is what they are trying to avoid.
It's not a requirement, but I have found it better to be safe than sorry.... There is nothing worse then having the officer ask if you have anything to declare, and when you say no, they tell you to pull into the garage for inspection anyway.... and when they find *1* item you can't PROVE you owned from before, they become obsessed with finding more.
Regards
Christian