I would put something on the back to stabilize the fabric for when they write on it. I will make one for a friend this summer. There are so many ideas. Questions that will impact your decision:
1. How big should the finished quilt be?
2. Do people sign or leave a message and sign (so how big should their writing surface be?)
3. How many blocks for people to sign? How many people/couples will need to sign?
4. Are you just using your wedding colors (2 color quilt) or scrap, etc.