A very fine grade of sandpaper placed under the block will 'grip' the fabric so it doesn't stretch while being written on. You could make a paper frame to place over the blocks, which only allows writing to happen 1/2" or so from the edges. That way you don't have to mark or tape each piece. These ideas will work best if someone is sitting at the table to set up the blocks for signing.
Congrats on the wedding!
Also, Sharpie markers are NOT permanent. They will be totally gone after a few washings. Always use pens that are made for fabric, like Pigma. Then be sure to heat set them with the iron.