A longarm quilter will not want to receive a quilt that has been basted. The three components (top, batting, and backing) should be completely separate. Generally 4" on each side is sufficient, and some longarmers are willing to work with 3" all around.
If a quilt back has vertical seams, these will build up in thickness when wrapped around the longarm rails. If the quilt is not very large, the buildup will be small and won't matter. Sometimes the quilt can be mounted at a 90 degree angle, so the seams will be horizontal rather than vertical, but that depends on the type of design being done.