Hi,
As a hand quilter I never handquilt in the ditch. Even if I machine quilt I use a decorative stitch or at least a zigzag that strattles the piecing seam line. For hand quilting if I am following the piecing lines it is 1/4 inch on either side of the stitching line. I do this particularly if the item was handpieced. That way it brings out the design and reenforces the stitching of the piecing.
I love the stencils called continious line. They make it much easier to quilt a design as you don't have to keep tying off or sliding under. I have used and continue to use anything I can find to mark and quilt with. I have a collection of cookie cutters that make great quilting patterns. I collect them as memontoes of my vacations and future quilting patterns. You can even use molds like they make for jello etc.