I would like to point out that you don't need to adjust RPM (prop pitch) manually. You can, but AH automatically adjusts your prop pitch with your throttle by default, so under normal conditions you can fly and fight just fine without messing with your prop pitch.
However, if you want to mess with your prop pitch, you can and it will have the proper effects. If I am trying to nurse my P-47 back home on a very small ammount of fuel, I pull the throttle back a bunch, and I manually cut my RPM's using the prop pitch controls to help me get maximum fuel endurance. Wether decreasing RPM is going to help you get maximum fuel efficiency or not is debatable and highly dependent on conditions though, so don't worry about it too much. Just throttling back works just fine in most situations. Also, if you wanted to glide really well with your throttle in idle for some reason, pull your prop pitch back all the way. I'm not sure how a dead engine would effect this, but if I get an engine hit and it dies, I assume it's not feathered and I manually pull my pitch down for maximum glide. This might be un-necessary, you'd have to test. I do it just in case if I'm trying to glide a wounded bird home.
