When the batting says 8-10", what I believe it means is that from any point in the quilt you shouldn't be able to move more than 8-10" without hitting a seam line. Quilting diagonal parallel lines would not meet that requirement.
However - I don't actually know whether your batting will shift, come apart, ball up, or otherwise misbehave. I personally wouldn't take the chance, especially if the quilt will be washed often.