I notice that most gunslingers play in interlock, does this mean that perception has any use? Or are Ranged Combat Tactics Still used w/ guns in interlock?

Still rely on RCT in interlock

Range Combat Tactics are for using guns. It doesn't matter if you are in or out interlock. Of course you have to select your guns inside interlock and not fight with self-defense or any other MA style, because then your MCT (Melee Combat Tactics) counts. Worst thing to a pure gunman that can happen is to get disarmed, since the abilities from the gunmen tree only enhance RCT (Range Combat Tactics).

I believe the explaination of Perception describes that it is useful for Martial Artists. Focus mentions something about the use of guns as well. This you can see in the window were you level up your attributes.

Focus gives points to RCT. Perception to MCT.

So, to answer your question: Yes, RCT is still being used inside interlock. Because it doesn't enhances your tactics out of interlock (free-fire) but because it enhances the use of guns.