I think the best thing to do in most cases is to post a link and suggest that next time, they do a forum search. Every time I tell somebody that the lyrics have been posted in a previous thread and they should do a forum search, somebody else comes in right after me and posts the lyrics a second time. If I post a link, it seems to happen less often. The most-often reqested songs are usually requested by people who are new here, and they need a little help.
What I don't like to see is people who tell requesters to look in some obscure printed source. I live in a metropolitan area with over a million people, and I have a better folk music reference library than all the libraries in town combined - and still there are many out-of-print reference books I don't have. I think it's best to assume that people who post requests don't have resources at their fingertips and may not have the Internet search skills we may have developed - so we should be as generous as possible.
-Joe Offer-