The rule I was taught was, where do you want your error to show up, at the top or the bottom?
If I am cutting out parts by cutting all the way through the material then I zero on the spoilboard because I always want the cut to go completely through regardless of thickness of the material. Not all pieces of 3/4" plywood are exactly 0.75" thick. If I zero on the surface and the material is thinner than 075" then I'm cutting into the spoilboard. If the material is thicker than 0.75" then my parts aren't cut all the way through and I waste a lot of time trying to get them out.
If I am V-Carving or carving 3D then I may zero of the material surface because otherwise the V-Carving may be too deep or too shallow. On 3D if I zeroed of the spoilboard and the material was a bit thinner than I thought I may end up missing some of the detail on the higher parts.
However, after doing this for a long time I prefer to stay with always zeroing on the spoilboard, and ALWAYS using calipers in several spots to determine the thickness of my material if I am V-Carving or doing 3D.
- Randall Newcomb