Also, if you do choose to comment something, and you see that the discussion it spawns is not being productive, or is negatively affecting the overall tone in the post, you can always delete or edit it down.
Sometimes the person you're commenting to is not in a mood or headspace for even a good-faith discussion, even if they're not actually venting, even though in another case someone else may have been open to discuss the point, and you can always gracefully retract a comment, or bow out of the discussion.