I don't have any hard and fast rules for how I use the voting system, but usually I upvote anything that I found insightful or that made me laugh. If a comment pulled a smile or a "huh" out of me, it's probably gonna get upvoted. It's sort of an easy and silent way of participating in a conversation when you don't have anything in particular to say. Kinda like smiling or laughing or making noises or gestures like you would do in a face-to-face conversation, I think--at least, that's what I sort of unconsciously equate it to.