Unless you live in a laboratory environment, keeping the level of humidification of your
guitar constant is virtually impossible. As a consequence, the top of your guitar will
rise and sink—as explained in Section
"First Things First: Proper Humidifiation"—even
if you stay pretty much within the recommended 40% to 60% humidity range.
Therefore, the action of your guitar will not always stay exactly where you originally
set it. On many guitars, the neck relief will not be very stable either. I have not been
able to correlate these fluctuations of the neck relief with humidity, temperature, or any
other outside influence. All I know is that it doesn't always stay in one place.
There is not very much you can do about this lack of stability of the setup. The bottom line is that you'll have to be able to play your guitar with the setup varying within certain limits. That's part of a guitar player's life.
If, on the other hand, you're a real stickler for an exact setup, like I am, you may be interested to hear how I deal with the problem: I check the setup every single time before playing. If it is outside of my comfort zone, which it is very frequently, I first adjust the neck relief, if necessary. Then I drop in one of a whole array of saddles of different heights that I carry around with me in my guitar case. That way, I have the neck relief and the thirteenth-fret action right, and the first fret action will come out right by itself, because the one thing that does not fluctuate is the nut slot depth. The whole thing is a big pain in the neck (no pun intended), but it's worth it for me.
There you have it, my take on acoustic steel string guitar setup. Good luck, and enjoy!