well, I am a long arm quilter, and 108 fabric isnt going to work with a quilt that measures 108-you need at least 4 inches on all sides in order to have enough backing to go around.
Also, I will say dont make a skimpy 1/4 inch seam in the back, either, i like to make a 1 inch seam in the back, and press it open, that way theres not alot of bulk.and thats personal preference i guess, one more thing- the seam should go horizontal in your quilt, not vertical, it DOES make a difference, hope this helps.
katz