Hi Dorothy. I am by no means an expert, but I'll give you my advice. Cut off the selvedge edges because they can be harder to quilt through. I would not serge the seams either. I was taught to sew my backing seams at 1/2" and press open to cut down on the bulk. Also, your seams should go up and down because there is less stress on them that way. Hope this helps. Maybe someone will come around and give different advice, but this is the way I do it, although I am just piecing most of my backs now. I just use 10.5 squares, or resize a block to a giant size for the back and add a large border around it. I don't buy fabric just for backing any more, although when I get the 2 Double Wedding Rings quilted that my mom made for me, I'll buy the wide muslin for those. Too much info, I know.
Joyce