Stick with what's in the POH or the operators manual for a specific engine and your butt will be covered.
Many POHs specify power settings where MP is greater than RPM/100.
I don't have one beside me but, for example, I seem to recall that one of the recommended cruise power settings for the BE55 is 24"/2200.
This is yet another area of aviation where urban myth rules! I too was told many years ago to "never operate the engine 'over-square'".
Dr 