Rhino Rails not "level" - do I need shims?

Hi folks,
looking for some wisdom here on racks for camper shells.
I added a pair of Rhino rails to my Leer 100XR last night, and noticed (a little too late) that they aren't horizontal left-right. The upper clamping surface slopes slightly to the outside.

Photo annotated here:



I'm planning on adding Prinsu-style racks on the rails, using something like the RCI bed riser or the Sherpa adjustable legs. But I'm wondering if I need to shim the mating surface between the rails and the rack legs.

I had assumed that it wouldn't be an issue, but snooping around some of the other trucks parked at the office I noticed that most had a much less pronounced slope, or had pretty-much horizontal surface on the rails.

Anyone been here before?