Storm, hickory showed the least amount of wear, over all the different materials I tested, including brass and nylon. The hickory rod barrely removed the bluing from the piece of early TC barrel - pretty sure it was 12L14 or whatever it is, it's soft.
I've never usd a centering device. My .69, which has shot 3 times more shots than any other rifle I have, shows just a touch of wear. Having such a large muzzle allows a good inspection. The tich of wear in the middle of one groove has not hurt accurcy one iota. It still shots 1" to 1 1/2" at 100 meters for 5 shots, rest shooting. This barrel has fired off close to 500 pounds of lead, betwen 6,500 and 7,000 shots, mostly loaded with it's tapered hickory rod. I have used a 1/2" nylon rod on occasion while at the range.
As in the thread I supplied the url for, I did have a .50 bullet barrel I used an unprotected fiberglass rod in. I had to shorten and recrown the barrel every 200 to 250 shots - like clock work. That is the number of rounds it took to double it's groups from 1" to 2" at 100yards, rest shooting.