Well, one may never be loving his motor too much...say once every several outings
This may avoid muds, sands building up inside the cooling passages & fittings..
Though I rarely noticed my friend flush his motors over all the years
...Merc 90/2stroke, Yammy 150/4 stroke, Merc 150/4 stroke...never had any cooling issue..
Had experienced a lot of clogging on that 90, all fixed by use of nylon tie or thin wire to clean out the telltale (outlet hole) from outside & inside...
Bigger motors could be more forgiving in this department I'd guess, as they have bigger everything which may not get clogged as often as the smaller ones..