If you have any left over, throw it away. Brake fluid is extremely hygroscopic. (It absorbs moisture from the air.) When it absorbs moisture, its boiling point is substantially decreased and its vapor pressure is increased. Water contaminated brake fluid is what causes a spongy and mushy feeling brake pedal.
You're not saving any money by keeping containers of brake fluid around your shop once the seal is broken.
Some fluids are more sensitive to this than others. FWIW, I've used Super Blue / Type 200 for almost 10 years in the Mini, have never thrown away a partially used container, have frequently bled using a container that was opened (and re-sealed) months earlier, and have never had a problem, including running very aggressive mountain weekends on track pads, and track days. I'm not saying it's not a problem for some people... but it's never been a problem for me.
The only time I've ever had a spongy pedal, was with some air in the ABS system.
same here with reusing opened container fluid. There's only so much moisture that can be absorbed from a half liter of air. I don't leave it sitting in the pressure bleeder, as that is open to the atmosphere when not in use.
If I have leftovers, I usually run those leftovers through the system first, to pre-flush reservoir and lines, then switch to content of a fresh can of ATE 200.
