One of my favorite quits is backed with white. The quilting (machine done) used a varigated thread in rainbow colors - I think the proper name was 'primary' colorway. The disign stands out wonderfully, and the thread was picked because of the fabrics chosen for the front.
Having said that, I usually back my quilts with a coordinating fabric, unless there is a compelling reason to use white/off-white. For a child, I try to use flannel, because the quilt won't slide off the bed so easily.