Anything goes, up to a point. Quite often when I get the blocks made but before I sew them together there will be one fabric that sticks out like a sore thumb and just has to be yanked. Another alternative when that happens is to add more of it in different places around the top. I like a border with lots of different fabrics in it, too. There's a book that I find helpful, Nickel Quilts & Borders: 7 Quilts & 260 Borders from 5-Inch Squares by Pat Speth and Roxie Speth.