although the amount of deflection mentioned by jonathan may seem small, it would be noticeable over a large machine base, or if your surfacing or engraving anything bigger than 500mm.

In his build, he shows you how to use epoxy to get it level, as close as you would need it.

I also fly RC planes. If you considering cutting wing profiles, you will notice even a small deflection. All depends on what your doing.

IMHO, for the sake of £ 45 for the expox, and a few hours ( no expencive tools needed ), do the resin thing, get it as accurate as possible.

Imagine doing a glider wing profile from foam, 1000mm long each, a small amount deflection on your rails would ruin the wing.