I wash almost 100% of my fabric. About the only time I do not, is if the amount of fabric in a kit is questionable, then I will leave it unwashed. I always unfold it and usually starch it as I am ironing it so it has the "sizing" feel of new fabric again and helps sooooo much with bias cuts (1/2 sqs., etc.) If I am planning to store the fabric for some time (ie--aging required!!), then I do not starch until ready to use.
DebraK, I wash extra wide for backings the same like 42". Basically wash it like you would your bedsheets. The bigger challenge is pressing it but I often fold it in half and then flip to get the other side. I have a bigger top (ironing board thingy) placed on my regular ironing board.