We talk about seam width, but you don't measure the seam, you measure the resultant patch:
http://www.quiltingboard.com/tutoria...ce-t89997.html
If you are making a top with squares all the same size, it doesn't matter. If you are making a pattern with multiple patches in your block, then it matters. If you are contributing a block to a group quilt, they are looking for accuracy so all the blocks fit together. Find out what size block they are looking for (unfinished) and make sure that yours measures this.