I can't answer for the UK, but with Canada, the official story is that if traveling to the US you have to carry a Canada Customs 'Y38' card which lists the serial numbers of all of your equipment. If you're stopped at the border when returning, the card verifies that all of the equipment was purchased in Canada. Similarly, if sending a piece of equipment to the US for repair, the repair lab has to clearly label the package "Canadian goods being returned to Canada" to avoid paying duty, etc.
Now, you've 'successfully' brought that piece of equipment back from the US... what you don't realize is that Canada Customs has five years to find out about you. If they come calling and you can't provide an original receipt, then you get to pay the full cost of the item (Cdn dollars), all of the duty and taxes, a nice fine and then they confiscate the equipment anyway. Now it's unlikely that Customs is going to come after you for something cheap, but if it's that cheap there's no advantage in the first place.
Is it worth it? Only you can decide.
Mike.