it is very relivant bc the base metal has a lot to do with the appearence of the metal that has been plated on top of it. this is especially true because the thickness of any plating on reps is minimal at best. what i think your are missing is that just bc 2 objects are plated with the same material doesnt mean they will look the same so someone who can spot the difference (dealers/engineers/metals workers etc) due to the differences in the base material, temperatures and checmicals involved in the process. the only real way around this is to plate the crap out of the case/band but then you are left with an object that wont look as refined. im taking a guess that most reps have up to aprox 5mils thick of whatever the last layer is. Keep in mind that 5mils is not very thick at all and is still translucent. therefore a 316LSS (10-14% nickel) plated object w/ rhodium @ 5 mils will apear darker than a 420SS bc the 420 will be brighter due to its lack of nickel content and its harder.
as far as dealers go, im sure different dealers have different sources who plate differently by using more or less rhodium. and the end of the day, reps are still reps.
rolex uses 440 (so do surgens) which contains no nickel and is easily machineable. 904L is not easily machined and is aprox 23-28% nickel. not good for jewerly bc many people are actually alergic to nickel. it causes skin rashes. not good for surgens
all the stainless steel info is from http://www.azom.com/materials.asp rolex using 440 grade SS is from google