The old rule of thumb is to keep your manifold pressure lower than your prop rpm such as crusing at 20" MAP and 2300 RPM. Also the same reason they say when you add power you go from right to left such as increasing your prop rpm to 2500 first and then adding throttle to get your MAP to 25" and the same with descent to ensure that you are always in that safe envelope they claim.

In actuality when you look into the POHs for most constant speed prop aircraft you will find many power settings where best performance and the factory reccomended setting requires an over square condition such as maybe having MAP at 25" and RPM at 2400.