Mike,
One of the first rifles I handloaded for after buying a Herter's press in the early 1970's was a Remington 700 in .270 Winchester. Bought a set of RCBS dies and used them for around 20 years before replacing them with a different brand--not because the RCBS dies were worn out, but because I "upgraded" to a pricier set of dies. This was shortly after I got my first bullet-concentricity gauge and the new dies would theoretically make straighter ammo. They didn't, and in fact the RCBS dies (I discovered) made VERY straight ammo with 150-grain Hornady Interlocks, which is probably why they shot so well in that 700.
In all that time I never cleaned any brass, and never found any difference in how the RCBS dies worked, even after loading an awful lot of ammo with 'em. In fact I shot the barrel out on that first .270 mostly because it made such pretty little groups. All that ammo was loaded with the RCBS dies, and they were still making very straight ammo with 150 Hornadys at the time of the "upgrade."
However, while the cases were sometimes a little tarnished, I couldn't call them "dirty," since most weren't ejected into the Montana dirt, but moved from the rifle's action to the ammo box the loaded rounds just came from. (The few that landed in the dirt usually did so while hunting, so were sometimes lost.) And of course I lubed the cases before resizing, so there was always a layer of Imperial wax, or something else, between the "dirty" brass and the inside of the die.
During that same period, some company sent me a case-cleaning machine with all the necessary stuff to make brass nice and shiny. I used it a few times and found the process a PITA, partly because I could have had the cases loaded up in the time the machine took to clean 'em.
The only time I've used it since was to shine up some new .416 Remington Magnum brass that had somehow gotten a little tarnished while stored in a box in my loading room. I'd sold the rifle so didn't need the brass, and making it shiny probably helped the sale. Other than that I haven't bothered, and have a bunch of other dies that still make very accurate ammo despite often resizing "unclean" cases.
