If you have people signing the finished quilt, or the finished top, you run a very real danger that someone will "mess up" and ruin the whole thing. The only time I think that would be the way to go is if there is a compelling reason to give the finished quilt away at the same time it is signed.
I would back squares with freezer paper, mask off the seam allowances in some way (someone suggested cutting the shape out of a piece of colored plastic, like a notebook folder, or you could use tape), make lots of extras for the cases when people want to start over.