Brass players get so used to working the spit valve that we tend to empty it willy-nilly without much thought about where and how to do it. While I do want to point out that it’s really just mostly water and not something to freak out about (get over it, string players!), I’m not insensitive to the fact that we should follow some etiquette regarding how and where to use our “water keys.”
Fortunately, in most rehearsal and performance situations I find myself in this isn’t really an issue. Auditoriums and rehearsal halls that regularly have brass performers in them are pretty much used to the little pools of spit, I mean, water in the brass sections. As long as you’re not emptying them right where people will step, it’s not worth worrying about. Be aware, though, that even just a little moisture can make a patch of floor slippery, so don’t empty your spit valve in areas where people are likely to be walking.
Trombonists, you will want to be aware of where your slide will be while playing and suggest to those woodwind players sitting in front of you that they may not want to put anything underneath. A little bit of water will drip off of your spit valve as you play and whatever music, purse, case, etc. the players in front of you put down will likely be “christened” by your spit valve during the course of the rehearsal.
There are some halls where emptying your spit valve will be more of an issue. A lot of the churches I perform at have carpeting down in the area where I sit and they are keen on keeping it looking nice with minimal effort. The Land of the Sky Symphonic Band keeps a set of small towels that we take to performances so the brass players can throw them down on the ground and empty their spit valve onto them. I keep a small towel in my trombone case for situations like this. When I’m rehearsing at someone’s home I will either use my towel or sometimes just empty my spit into my case. I wouldn’t intentionally spill a glass of water on someone’s floor, so I will go out of my way to avoid emptying my spit valve unless they specifically give me permission.
When performing in a solo situation, either in front of an ensemble or in a recital situation, I think it’s classy to not call too much attention to emptying out your spit. I will turn slightly to the side and without any fuss empty my spit out behind me. It’s more subtle and less distracting than unceremoniously blowing all the water out in full view of the audience, many who may not have any idea why you’re doing that. This also has the benefit that it keeps the spit away from where most people are likely to walk, especially important if you’re sharing the recital with other performers who may slip on your puddle if you’re not careful. Let alone the conductor, if you’re performing a concerto with an ensemble.
Beyond that, it’s mostly just a matter of being aware of your surroundings and being polite. While I admit it’s fun to sometimes make fun of the string players and woodwind players aversions to the puddles we leave on the floor, they typically understand it’s part of the price we pay for playing the instrument we chose and will leave us alone about it as long as we keep it confined to our area. Now can we please just get the woodwind players to start being more careful about where they leave their broken reeds and string players to quit spreading their rosin dust around? Those are much more serious issues than a little water on the floor!