Upvote what you want to encourage. I've upvoted most of the replies here; this post seems to have got an unusually good ratio of valuable replies (admittedly those replies are somewhat repetitive)

I think the range of appropriate policies is roughly from "upvote everything you regard as, on balance, a net positive contribution to LW" to "upvote things you regard as exceptionally high quality". Indiscriminate upvoting (or downvoting) of everything in a particular discussion would be frowned on a bit, at least by me and anyone else whose brain happens to be identical to mine, but it's not forbidden or anything.