I make sure to get insurance every time I go to the states. This past thanksgiving I was riding Galbraith with an American cousin, I went flying over the bars on a green trail and broke one of my ribs. I didn't want to test out my insurance but I was sure glad to have it, and I missed a month of work from that crash.
I did get asked when coming back when and where I had bought my bike. I told the border guards and informed them they could call the bike shop I purchased it from for further confirmation, they didn't have any other questions after that.