I agree with Rodney. I would not build without a razor saw. Actually, mine is an Xacto, and it seems to do the job fine, but maybe I'm not tuned in to the fine points of razor saw quality.

My razor saw (and I think most) have the teeth oriented so it cuts on the pull stroke. That makes it very easy to make very accurate cuts. On the push, you can line up the blade very accurately and the teeth dig in just enough to make a path for the pull where the real cutting is done.