OK- there's clearly a consensus among the posters that placing a bit of grease on the neck tenon is a morally bankrupt practice.... but I don't see why. It is, at least on tenor, alto, and baritone, an easily accessible location. There's no earthly reason why it should accumulate undue crud; I wipe the horn lightly with a rag each time I put it away and that includes about 7.23 seconds to run the rag around the inside/outside of the body socket and neck tenon. While no substitute for a good snug fit it certainly ensures that last bit of sealing perfection and, assuming one isn't slovenly enough to allow it to fill up with dirt, probably reduces rather than speeds up wear. It's not in an area where there's any risk of contaminating pads or mechanism- heck- not even clothing. Trumpets use the stuff on carefully machined slides with at least as close tolerances and there's no issue. I've been doing it for the past forty odd years and have had no issues. I'm certainly willing to change if there's some good reason but, "It's just not done my dear chap." isn't persuasive.
So what's the deal?