After early seasons rains, I hear a lot of metal-on-metal grinding sounds when cyclists come to a stop.
This is because as you brake, small shavings come off the rim and embed themselves in the soft rubber brake shoes. This (a) decreases brake efficiency, and (b) wears the rims out quicker due to the metal in the brake pads grinding on the metal rim.
Solution: Every month, take a small tool with a sharp edge on it (I use an old small screwdriver that I have reprofiled to a sharp edge on each side of the spade, but you can use anything else such as a small knife blade, the point of a small pick, etc) to dig the metal shavings out of the shoes. This will result in more efficient and noiseless braking, and extended rim life. A good set of Dura Ace wheels will set you back close to $1,000 so all the better reason to make them last as long as possible.
Remove the wheels from the bicycle and carefully dig out the shavings. Being careful not to nick any carbon of the frame.
For those more adventurous, after digging out the shavings, tune up the pads with a file. Try to smooth out the pads so that they are flat and even again. [Notice the attached picture where the pads have a groove cut in them]. Again, be careful not to knick the carbon with the file.