You can also spread the stem without any chance of leaving a mark by inseting a large phillips screwdriver in the bolt hole and pulling back.

Obviously, if Nitto makes a tool to spread their stems it's okay to do it.

The problem it not getting the bar through the stem. It's done the way Chombi desribes and I think that anyone who has spent much time working on bikes knows it.. The problem is when you get to the larger clamp area. You often have to spread the stem or leave marks on the bar at that point.