No point in tightening up the lid of a half full bottle, as the fluid will absorb the moisture in the air part
To absorb a significant amount of water, the fluid would have to be stored in an open container. If the container is sealed, there will be a tiny amount of moisture in the air at the top. This is not enough to be a problem. Once that has been absorbed, there’s no more moisture to absorb. The only way it can absorb enough moisture to be a problem is if the container is open and the moisture content of the air is constantly being equalized with the atmosphere.
If I was racing DH, I’d flush my brake fluid at least every six months and use an unopened bottle each time just to eliminate any remote chance of a problem. For most people, if you screw the cap back on the bottle, you aren’t going to have boiling brake fluid. The absorbing water thing is massively exaggerated.