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.