Suspicion on the US side is high as the Canadian dollar has slipped recently (down to around 65-66 cents US) and hence US citizens are tempted to buy lots of stuff here and take it back without declaring.

Actually if you travel to Canada frequently enough (and enter through Toronto, Montreal or Vancouver) then you should get an INSPASS for covering the US side (costs USD$20) - it doesn't preclude them from questioning you but generally they don't - you just go to an ATM-style machine and scan your hand. It is also being introduced (maybe already introduced) at other Canadian airport as well

about the Canadian side I've never had any problem - including 3 weeks ago when I moved my stuff back from Seattle to Toronto and had two huge suitcases + carry-on's and a notebook computer.