Top level dust washers is extremely common on cabs you can look in brochure flyers or arcades in Japan they are used all the time unless its missing. Only a certain sub set of cabs that I can recall off the top of my head may not have had dust washers laying on the top of the CP. Shaft gaurds is another matter entirely.
If you had a super rare CP or didnt want to scuff your CP then I feel like at that point you might as well have that panel repro'd then store the original for safe keeping.
But yes if you put thousands of play time hours on a panel then the dust washer will likely rub on the panel and scuff the area sure. I guess maybe a solution to that might be adding a very very light layer of some kind of oil on the bottom side of the dust washer but I dont think that would do much tbh.
I cant think of or have any list of mounting examples, the way you mounted yours looks fine I know some panels will have you actually remove the JLF plate and mount it directly to the panel or a special sub panel like maybe what taito liked to do with their Egrets, at least if memory serves correctly. Thats why Sanwa has those two holes on the ends.
Top washer or not you DEFINITELY want the lower washer ideally, it keeps dust and grime out of the lever which is bad news for the grease in there if its contaminated with long enough play hours.
, it's a repro 
