Bringing back new items to USA from Canada

I cross the Canada/US border regularly - about once a month for the past five years.

You aren't required to declare every little thing you bring with you, if it's reasonable to have it with you. I have my laptop with me almost every time I cross, and I have yet to declare it once. It's personal property, I'm allowed to have it, and I'm not leaving it in the US (I live in Canada), so there's no need to declare it. (The same is true if you live in the US and are entering Canada.)

The only things you need to declare when entering a foreign country are things you are leaving in the foreign country (e.g. if you are mailing something, giving it to someone or selling it), business property (samples, stock, promotional materials), and anything that typically has import restrictions (food, controlled medications, and such). When entering your country of residence, you need to declare anything you didn't have when you left (i.e. anything acquired out of the country), plus restricted goods like food (even if acquired in your home country; e.g. the US won't let you re-import citrus fruits you exported). If in doubt, declare. You can always ask if you're not sure. "I bring a pair of laptop computers with me when I travel here. Do you guys need me to declare them whenever I come?"

Twice, traveling to the US, I brought old retrocomputers (Sun SPARCstations) that I was giving to fellow retrocomputing enthusiasts. These I did have to declare, since I was leaving them behind. Given the age and low value, both situations were fine although the first time I did it, the officer wanted to see the computer (but he didn't look at it very long).