I think I’ve settled on the latter. Disagreement is maybe best communicated by the absence of an upvote? And downvotes work best when they signal something that is just off base, and while not reportable, is not appreciated at a broad cultural level.
I think of it like a digital facial expression. Upvoting is akin to smiling or conveying some other positive emotion like affirmation or understanding (even if the subject matter is inherently negative). Downvote is the opposite- someone says something irl that makes you frown or grimmace, or you know they’re telling you a lie, your expression can convey that without a single word. Here, downvote.
There’s no real etiquette - if you feel like you want to give an up or down vote, just do it and don’t put too much thought into it.