If I'm looking for a phrase (not just a word) that expresses a specific concept, should I still use the tag
Edit April 12th 2011: we now have phrase-requests. Let's see how it works out in practice. The original answer below should be ignored for the time being.
Since there is no "phrase-request" (and I had to fight for keeping the single-word-request), I would say that your best bet right now is to tag the question with both single-word-request and phrases. At least that's what I do in those cases.
First of all, that makes it easy to search for this type of questions (enabling straightforward retagging in case "phrase-request" does get introduced).
And secondly, the single-word-request doesn't do any harm, as
- there are many "actual" single-word-requests to which the answer is "there is no single word for that, but here's a phrase", and
- it might turn out that there actually is a single word for the concept you describe.