I use the fabric softener sheets as my backing...the used ones......they can be cut square or used as they come, as rectangles and I have used them both ways. They still smell good, and they are so light weight, it doesn't make your quilt so heavy - they wash just fine - I have used these a lot and have never had any problems.
I have done string quilts in scrappy, but I have also done them is one color patterns....like using many stripes, all with green prints of one kind and another............one of the prettiest ones I did was all in yellow prints... I had it at Project LInus and everybody was asking how did you do that??? I have also done them is all solids..that makes an interesting one too, all different shades of all the colors...you can do them with all the strips the same width or all the strips different widths........I don't think there is a wrong way to do it..........