I have not made a signature quilt. However I have read comments from many other people who have made them. The general consensus seems to be to pre-wash the fabric that will be signed, then cut it into squares. I would make them a bit larger than you will need for the quilt, to give me the option of centering the signature if necessary. Make cardboard templates to go over the signature squares with an opening the size of the final block (so no one will sign into the seam allowances). Provide pens for signing, and someone to explain how to use the template over the fabric. Make more fabric squares than you will actually need, because some will inevitably be ruined during signing. Either iron the squares to freezer paper or place fine sandpaper under the fabric to keep it in place while the guest signs. I would also provide a practice scrap that guests can scribble on to get the feel of writing on fabric before they sign the real thing. Pigma pens, or other fabric pens must be used. Regular ink pens will fade or run.
Even steam pressing the jelly roll strips can cause shrinkage. I would press it if necessary if with a dry iron, but I
would not wash or steam press it.
This video also has some helpful tips.
https://www.youtube.com/watch?v=FxJYOFvmXS4