There's always interesting banter (sometimes personally contentious) about this on the other site and it usually involves Strads. Apparently it's an option (brass or nylon guides) or used to be. Anyway, it seems to revolve around the mass/resonance theory that has given us heavy valve caps, sheet metal bracing and "heavy" horns. Some say there isn't enough mass to change the sound perceptively, others swear by it. Someone asked if there were any "scientific" analysis done and Mark Curry said and I quote, "Documented? I don't know if anyone has studied this and published results". That was June of this year. So, it still seems to revolve around an individuals perception. Try the washer in the 3rd valve cap (bottom of course) and see what it does for you before you spend real money switching guides around.