Don't know how much could build up on a spinner, but it is not rotating at "high revs" as you might think, simply because propellors become inefficient at higher revs, so are geared down to around 2000 revs. I suspect it would be impossible for enough to build up on a spinner to become a problem as it would shake off as soon as it got to any thickness ... but I've no doubt somebody will correct me.
UFO