I have a home system, not a true longarm, but the same principle applies--I start at the bottom, top or side--depends on how I plan to quilt it and work my way to the opposite side. I would only baste a machine quilted quilt if I was doing a very intricate design that absolutely had to be perfect--like whole cloth--or if all the material & batting has been pre-shrunk--which mine never are because I like the wrinkly look. For an art quilt or wallhanging, though, you might want to pre-baste so your calculations can be exact for the finished project.
One thing I do baste are the borders if I'm working top to bottom or vice-versa. The borders will get all wonky if you don't. I just stick flower head pins every 4-6 inches up the border though--nothing fancy. I've got a quilt on the frame right now that is a diagonal setting with motifs in the setting triangles so some of those were pin basted as I was doing other areas because I had to turn the quilt to do those areas.
Clear as mud?
Beverly