I have a longarm, but I do not like to add the binding while the quilt is on the frame. Like was mentioned above, I square up the quilt after it is quilted so it is not possible to have attached the binding first.
I think it is ok to use this method for charity quilts. As I say this, I am cringing!! I do feel we should do our best work on not only on our own quilts, but on charity quilts as well.