Nuts to that, that's getting close to Quilt Police territory, in my opinion. It all depends on the quilt. I have made many quilts where adding black or white would have been completely inappropriate and jarring to the look of the quilt.
One of the things I do consider when making a quilt, especially a scrap quilt, is value. I think a quilt "sparkles" more when it has a few light fabrics scattered around for contrast. If it's all medium value fabrics, it can end up looking dull and drab.