I think this should be "never downvote".

Seems to me that a downvote would associate negative valance with both the act of posting on LW and with whatever their specific mistake is, with the latter being stronger. So no vote and a comment with a mixture of praise and criticism is probably the stronger play if you're looking to improve someone's writing or fix some technical mistake while keeping them as a contributor, but a downvote is still effective if all you care about is seeing fewer posts of that kind.