I think everyone will agree with you, selecting a lower gear and using engine braking going down steep grades is appropriate.
I believe what some are asking is will the transmission suffer excessive wear by shifting it regulary, gear through gear, manually.
My understanding is that up-shifting, such as 2 to 3, 3 to 4 etc isn't hard on the transmission. You will probably not get as good gas mileage, and as long as there are not severe speed mismatches between gears, no problem.
But it is my understanding that pulling the gear selector from say 4 to 3, or 3 to 2 is hard on the clutches. Actually similiar to a manual transmission in that the differences in gear speeds must be compensated for. It is this type of use where excessive wear comes about.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)