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.
This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2008/01/07/brake-fluid-etiquette