Unfortunately I gave away my copy of the C404 POH many years ago, but I do not recall any advice from Cessna to "reduce speed and select flap 10 before starting descent", or such complex instructions re engine management. No flight engineer, so what attention would our pilot be paying to the outside world if fixated on manifold pressure?. Not saying that is is not worthy of some care, but really - less than an inch per minute? Is that done by use of a stopwatch or, in reality could we be trusted with general advice to gradually reduce it, rather than make rapid changes, and heed the other general advice pertaining to geared props: to keep them positively loaded until near landing?
That 'checklist' is more like a (misguided) instruction manual for student pilots. Hanging flap out before you need it is a baaad idea, not only from a cost perspective, but many airframes are less tolerant of turbulence with flap.
When operating any aircraft: time/drag = fuel = money, so whoever wrote that checklist was not flying in the real world..
However, we all know that older tech turbo charged engines do need gentle run down time. The more of that done while taxying in, the better, as air-cooled engines don't take kindly to idling while stationary on a hot day. The checklist-cum-how-to-fly book quoted above is a bit light-on with sound advice, but big on bullsh!t.