Talking about a NA engine now, my own is an IO520.
The last few percent of the throttle movement opens a 'fuel enrichment valve' that allows extra fuel to the engine. If you reduce MP just a bit, say from 29" to 25", you will see the CHT start to rise quickly because this valve closes. Keeping the throttle full open during the climb in an NA aircraft will keep CHT lower and the MP will fall by itself.
Reducing MP is totally unnecessary and even harmful in my situation, reducing RPM is optional and only for noise reduction on the ground. Once established in the climb, say from 4000 ft onwards I will lean to keep a constant EGT during climb.
There is a lot of bad or simplified advice floating around from old days when training had to be 'expedited' or due to lack of proper engine instrumentation.
I have flown a turbocharged twin for 15 years and there was a published climb power with given MP, RPM and fuel flow in the POH.