I wouldn't use one on the RD if there's an adjuster on the RD itself. I would use one on the FD as they allow to precisely fine tune the derailleur whenever you need.

Having to stop to detach the cable, find the correct position and clamp it again because something has moved a little is a PITA. Cables stretch, and an in-line adjuster allows you to tweak the derailleur even while you're pedaling if necessary.