I used to prefer prints for the back, but as my longarming is improving (at least I think it is) I find that I like to show it off on the back of the quilt, so I am using more solids. I also have used lots of heavily pieced backings, in order to use extra blocks or fabric left over from the front, or just because. (And I realize that is a contradiction, because with the heavily pieced backing my quilting really doesn't show much on the back.) I usually try to orient the quilt on the longarm so that the major seams go across, but not always. If the quilt isn't too big I can always turn it sideways to make the seams go across, if that is what I want. I usually have similar threads (in color and weight) in top and bottom, but again not always. I agree with feline fanatic that you should ask your longarmer what she prefers (and why), and if you prefer something different (that she can't work with) then you should find another longarmer.