I am member of many forums in internet built in vBulletin.
I have seen those forum that if an user likes any thread, then give some reputation to that user. Its kind of a culture in Forums to encourage a poster to post more and quality posts.
But in golpo, I haven't seen this culture.
But if we do so I think its a great encouragement.
so give rep. to other user if you like his/her post.