The operator should attempt to minimize themselves and their instruments as being distractions to the main event (fiddles, principal oboe, and so forth).
To me, that means "as many water keys as necessary to avoid unnecessary motion, unnecessary noises, and accidents".
I've just installed the 8th one on this instrument.
I've spent more time with it lately, and discovered that water (with no outlet - other than twirling or removing the slide) significantly collects in the upper #3.
This is my second UPPER RETURN water key, as I've had one on the #1 slide for quite some time.
Removing water from any place on this instrument involves no more than depressing a valve and a water key, or those two plus a subtle tilt of the instrument.
bloke "you know: trumpet players kicking over mutes, tuba players dropping slides on the floor or (fists-a-flying), accidentally spilling spit on the bass trombonist"

If still have some 5th linkage issues to resolve with this instrument (which - as of this week - have been solved in my mind, and the solution - for improvement - is simple). Otherwise (whenever I feel as though I have the time/energy...LOL as if...) it's ready to polish and shoot with clear.
Upper #3 floats as smoothly as upper #1. Per typical, when I make provisions for something (once an instrument is completed) I find that it's not needed. Serendipitously, all four of the 2-3 combination pitches are happy with the 3rd slide remaining in one spot.
Holton BB-flat (same body/bell as 32"-tall 4/4 York BB-flat w/19" bell) - it's a wonderful li'l thing.
When I redo the 5th linkage, I might (if I remember) dig up this thread.
