The Longarmers that I have dealt with ask for it to be horizontal with 1/2" seam, pressed open.
(agree with PaperPrincess, that shouldn't be an issue, as they can load it with the 90° turn.)
As you can see there are different thoughts as to what to do with a pieced backing.
.... you really would be best to speak directly with your LA'er and do it the way s/he wants done!