Well, we've gotten the "official" word, but I can't resist adding my two cents.

In general, if I found it somewhere here, I put a link to where I found it.

If I found it "offsite" I put lyrics in the forum if the song is clearly a "folk song" or if the question showed up more than once in my searches of the mudcat (a couple of the MASH songs, for example.) I feel that even offbase requests should be answered and if the question shows up multiple times, well, it deserves a notation that actually answers it consistently. I try to provide the link, too, if the site has lots of lyrics. I don't necessarily expect that sort of song to end up in the DT, but once it's in the forum, it's available. And some "commercial" songs of the past have clearly drifted into the "folk" category via the folk process, so a hundred years from now it might be fun to watch the process in action in old messages.

*But* if the place where I found the lyrics is an "official" site -- Rod McKuen lyrics at Rod McKuen's website, for example -- then I just put the link. I'm a little chary of reposting the lyrics in that case, although I'm not sure I can consistently explain why.

On the whole, I know that if I'm asking the question about lyrics, what I really want is a clear answer. And usually, that means lyrics.