Bear's advice is spot on. I wish every person who asks me to longarm their quilt would sew that staystitching line. It really helps stabilize those seams on the edges.
Before you trim your quilt, decide how wide your binding will be and where you will be sewing the binding to the front of the quilt. Then you can measure that far away from your anticipated seam line and trim. It does not matter if the batting and backing are slightly larger than the top of the quilt. They will be covered by the binding.