any time. they make a HUGE difference in sound and ease of hand pulling and of coarse no wear on the tub. they build them so you can reverse them (back to front/front to back) after they wear down.
after trying them on one sled, i installed them on all my sleds and won't be w/o them. i don't drag any thing across cement or asphalt on purpose tho with them. they will get destroyed, but can be replaced cheaper than the sled.