When my fretboard is straight and set-up is right (single truss rod), even if there's no buzzes or anything, I can "feel" it needs a tweaking, in my fingers. I can't "see" a concave fretboard, but just that little tighten, and it's easier to play. And then at some point, due to atmosphere changes (New Jersey has been really up-and-down this Autumn), I get that buzz, and a little loosen, and voila. My experience.
And I have to add, this has been almost day-to-day!

When the steel truss rod is adjusted to a set position and the strings don't buzz, the instrument is very playable. Atmospheric changes (i.e. temperature and humidity) cause the instrument to slightly expand or contract. If the instrument is expanding, the fretboard will go in the convex direction which will cause the strings to buzz. So, loosening the truss rod on a single rod system is the proper thing to do. This will cause the fretboard to move towards the concave direction. Ideally, we are looking for a flat fretboard but sometimes a bit of concave is needed for best playability.

The Railboard uses a different system. The truss rod is divided at the middle of the fretboard. It's setup so that it can push or pull in either direction. (i.e. from either end of the fretboard in relation to the center of the fretboard). I didn't understand this when I first saw it. I thought that there would be enough string tension that some truss rod counter tension would always be required. I was shocked to discover that I had to extend both parts of the divided truss rod to create a slightly concave fretboard.

With no truss rod pressures on either side, my Railboard was already perfectly flat (with all of the strings under proper tuning tension). This meant that the aluminum Railboard was unaffected by the string tensions. However, for best playability on my Railboard, a slightly concave fretboard worked best. It will be different for everyone because not all Sticks are created equally but they have the ability to be tuned.

The 'fixed in the middle, adjust at each end' truss rod is what wooden & bamboo Sticks have been fitted with too since mid-2009.

See: https://stick.com/about/history/timeline/