I must admit I was concerned with abrasion and always hung the tarp under the ridgeline. However, when I got my HG 4S Cuben Fiber tarp, I couldn't get it to pitch tight so I put the tarp over the ridgeline.
I spent the better part of two nights watching that ridgeline to see if it was rubbing the tarp. I never saw any movement of the ridgeline, even in winds up to 20 mph.
I am no longer concerned with tarp abrasion from hanging the tarp over the ridgeline. If I see that happening, I'll use Knotty's suggestion and put some slack in the ridgeline.