You definitely do not want to toss a quilt top (one that is not quilted) into the washing machine. It's not just strings from the border that you would have to deal with; some fabrics may fray so much that the seams become weakened. Also, unless you prewashed all of the fabrics, the quilt top can distort so much (because it has not yet been quilted to a batting) it can become a nightmare to try to get it to lie flat again. If you really have to wash the quilt top, I would baste it to muslin first to protect the seam allowances, wash in cold water on delicate, and then dry flat (because dryer heat can shrink some fabrics a lot).