Adding a super just to add one is counter productive. In determining whether or not to super you need to have the answer to several questions:
1. What's the current comb build out of the hive?
2. Is the comb and frames at least 80% full, drawn, and covered with bees?
3. What has the activity of the hive been like?
4. What is the per cent of brood in the comb already built?
5. What's the queens laying pattern?
6. If the hive is not developing like it should, what could be the reason?
7. Did the bees supercede the queen shortly after being installed?
8. Is there some reason I should still be feeding the bees (weather, lack of nector, etc.)?
I add the last one because that seems to be a more common development these days. When you've answered all the questions in the affirmative then super it.