Brake fluid "eats" paint. Although widely used for many decades, brake fluid is nasty stuff that — given the chance — will spill onto your paint and strip it off. Thousands of unhappy do-it-yourselfers have had the unfortunate experience of "topping off" the master cylinder and spilling a little fluid on the firewall, fender or cowl area under the windshield wipers. Later they've discovered damaged paint that couldn't be buffed out or repaired without a touchup.
A good habit to get into is to wrap the brake fluid container with a soft paper towel or rag, extending the material close to the pour spout. Then, if any drops fall they will be absorbed by the towel. If your master cylinder is hard to reach with the container's spout, place a large screwdriver (cleaned, of course) into the reservoir and place the spout on the screwdriver's shaft. Slowly pour and the fluid will creep along the shaft. When finished filling, seal the container tightly and throw away the towel or rag to prevent its touching the car's finish later. Just because it dries off doesn't mean the residue won't harm the paint.
Oh, and that opened container? Well, if it's less than half full don't keep it around more than 6 months. It will have absorbed too much moisture.