It depends on what aircraft your flying.  I think you are taking about constant speed propellers, in cruise it should be a little bit less than in climb (as to take a bigger bite of the air).  Manifold pressure should be a few inches less as well.  But it really depends on your aircraft.  Higher the RPM and manifold pressure gives you better performance, however its all about economy (and minimal stress to your engine).  I don't fly constant speed propellers personally, except in FS2004  , so im sure theres other people who can make a better explanation than I.

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That's a good point(sim world vs real world). Who wants to fly slower when they don't have to?
Another factor is the fact the fuel burn dynamics programmed into the sim are marginal at best. The Baron is a good one to experiment with. On a real aircraft with fuel flow meters, you will notice a change in fuel flow, not only with change  in MP but with changes in prop speed, mixture, altitude and angle of attack(none of which are reproduced accurately in the sim). I used to fly a Chieftain(the real one not the sim)and there are specified numbers in the operating manual for high speed OR long range cruise and everything in between.
In the sim I fly at max power most of the time. Why not?
In the real aircraft one of the first lessons that you'll learn is that pushing the limits of engines and airframe will usually yield minimal results.  I'll give you a good example:
When you are really "pushing it" in the Chieftain, you'll maintain 170-180kts. That's three miles per minute. Long range cruise for the airplane is more in the neighborhood of 150kts or two and half miles a minute.
On a 300 mile trip that equates to one hour and forty minutes vs. two hours even. Of course that's if you maintain those speeds from takeoff to landing and you don't do that!(unless maybe you ARE in the sim). From many years of experience I can tell you that when you factor in the climb, descent and entry into the arrival environment, that 20 minute difference is reduced to a ten minute difference or maybe even less at really busy airports.
Now here's the real big element. Long range cruise yields about 19 gallons per hour, per engine. Max power burn can exceed 26 per side.
1.6hrs = 83.2gal@high cruise   2.0hrs=76gal@long range cruise.......or about a $21 difference when using the current price average in the US.
If you think 10minutes is worth an extra twenty bucks then firewall those suckers! If you're flying for a penny pinching, charter outfit, you may want to pull it back to save some gas, the engines, the airframe and perhaps have the fuel necessary to shoot an extra approach or two if the destination airport is weathered in. 

it helps save on fuel when your reduce your prop rpm

Actually, it seems to work in the sim too... from my observations... Like the Beech King Air 350, you pull your props back your fuel flow goes down

Having a constant-speed prop is like having a transmission. Basically the idea is to get more thrust with less manifold pressure. In real life, this saves gas and wear and tear without having to go slower. So the limits apply to avoiding too much manifold pressure for a given rpm. All such equipped aircraft have specific settings of throttle and prop pitch for specific flight modes.
  As a general rule of thumb, and mind you this is second-hand, in RL constant-speed (variable pitch) prop-equipped airplanes, keep things "square" or under... eh, on second thought, forget it; this guy'll explain it better...
http://www.avweb.com/news/airman/184483-1.html

Pitch controls engine load (resulting in temperature)... Anyone that has the Yak-12 knows that.

Running 600-700mbar pressure with full pitch gives almost the same speed as running at the same power setting at 50% pitch