Having done remodeling for a number of years, I had the same thought. I'm planning on using Blandex (chipboard) for my forms. It's perfectly flat (plus, I seem to remember seeing a picture of chipboard forms somewhere). I would go with the thicker (not actually 3/4, I think it's 11/16). That should also make stapling easier, since 1/2 inch isn't much and there are often gaps in the middle of plywood.