how do you choose when to let your pedantry loose, and which hairs to split?

I only fight for or against usages when I know I'm right.

Seriously, this one started out as a tweak of the noses of prescriptivists who will invoke understood in one context and totally ignore it in others.

All seriousness aside, I *do believe that we reinvent grammar every generation. I also believe that some people are less capable than others of relearning, in a formal setting, the speech patterns they developed when learning the language on their own.