I like them straight up with the NRS frames. It may be an aesthetic thing with the NRS oar towers because I’ve used them at around 20 degrees for most of my commercial guiding.
Unfortunately I have no good explanation for the angle they should be. My take is that anywhere between 0 degrees and 20 degrees from vertical should be fine and the exact angle is not a huge deal.
There are some good photos in this article that show the 20-ish degree angle: https://www.nwrafting.com/river-equipment/oar-locks-vs-pins-and-clips.