I booked a non-refundable one way ticket (cheapest available) to Canada, and a refundable one-way business class ticket (it cost over 4000$) back to Japan in 5 months' time. I refunded it the day after I arrived in Canada. I did worry about whether or not it would look suspicious that I booked two separate, one-way tickets, one of them fully refundable, but they did not ask to see the ticket. But that's up to luck, I guess. I had a very easy time at the border (no questions asked except how much money I have for my stay. The immigration officer did not even ask if I am applying for PR (I had all my documents ready to show) and just stamped it and waved me on my way, so I did not bother to stop her to tell her I applied for PR. As long as she says she is "visiting" it's ok - the choice of words is important here.
I extended my visitor status after 5 months - I asked for, and got, a 1 year extension. However, the next month I got my COPR, so my husband and I drove to the US, stayed for a few hours, drove back and I "landed" as a PR. If you choose not to enter the US but turn around at the border, make sure you get a paper from the US side, I forgot what it was called - administrative refusal? or something like that, something that shows that you (your wife) voluntarily chose to withdraw her entry into the US and is turning back. I didn't do it (didn't turn around at the border), so I don't remember why it's important, but I think I heard that getting this piece of paper shows that you (she) chose not to enter the US and that she wasn't refused entry or something like that. This might not be relevant to you guys anyway - like us, you can just cross the border, have lunch or something, and come back. It's people who can't enter the US without applying for a visa that this applies to.