I set mine so they contact the padding on the roof (ceiling) so they don't move while the car is running. Would kinda suck if I did all that work and they rattled. And mine is a '72 so it didn't have all the holes in the edges-only the two sets at the back. I assumed the idea was to have each bow perpendicular to the roofline so as I was dry fitting I picked the holes in the back that made the bows look straightest. Having to drill the rest meant I could put the bow right where I wanted it.
My journey, if it helps.
