I do not use open bottles of brake fluid, if the vehicle I'm working on needs brake fluid they can buy a sealed bottle for 10$. To my knowledge most brake systems are not sealed, they are usually vented at the reservoir cap otherwise the fluid level would not drop if you cannot let air into the system.. Which is where the moisture enters the system. They make brake fluid testers that measure water content of the fluid, 3% water and the system requires fluid replacement. I change my brake fluid when my fluid tester says my fluid fails. the people that don't change their brake fluid usually regret it when they blow a brake line and they are forced to replace a working caliper just because the bleeder snapped or stripped.
What do you do with the brake fluid you don't use when topping off a master cylinder? Do you throw it away? or set it on the shelf to top off the next one? If you are that strict on the use of brake fluid why not save it for the next car or to top off a system and test it with your tester and use it if it is good according to the tester? I have never used a tester how often is a fluid change needed when checked with a tester? Is the interval the same in the rainy state of Washington or humid state of Florida as it is in Az or Southern CA?
Being in the automotive repair business for a whole career I never had anyone bring a car to me for a brake fluid change. I have used a power bleeder that replaced fluid when changing a component but that was a rare occurrence. At 60 years old and owning a bunch of vehicles I have never changed the brake fluid in any of them just to change the fluid. I'm not saying that you shouldn't if you feel the need to do so and it absolutely can't hurt anything as long as you use the proper fluid for the vehicle when changing. I'll just keep doing things the way I have for all theses years and will likely never have an issue just like in my first 60 years.
any venting for the MC is going on above the rubber seal on the cap. The flex in the rubber seal movement does the rest. Moisture thru a cap vent never gets to the fluid.