What dictates the prop RPM used during cruise? I've seen on numerous different constant-speed propeller aircraft and different pilots using different RPMs during the cruise phase on a propeller aircraft. What dictates the RPM used during normal cruise operations, and why is that the case? Often there are multiple possible throttle/RPM combinations that deliver the same power, so where would you pick one or the other?
"Dictate" is the wrong word.  Within the ranges listed in the POH, the pilot may select rpm/MP settings as he pleases to meet other criteria.   

Charles Lindbergh instructed many WWII pilots in a technique to extend range, by flying with low rpm and high manifold pressure.  This extended range by reducing the "pumping losses" required to maintain low manifold pressure.  Over long distances this technique did extend range considerably.  

Secondly, lower rpm reduces noise - remember that the propeller is the largest contributor to a propeller-driven plane's noise signature.  This is important in the terminal environment where the noise can affect the ground, but it also affects in-cockpit noise regardless of altitude.  

This answer does not contradict the former answer, but it extends it to point out that there are other important factors that the pilot should consider in selection of engine operation parameters.