There are even badges for it. Necromancer, Revival, Excavator, Archaeologist all reward you for going back to old posts and doing something (necromancer and revival for answering and excavator and archaeologist for editing.
If there isn't a good answer on an old question or you can do better than any previous answer then please provide one. SE questions are designed to be a lasting thing on the Internet and thus we should spend at least a little bit of time maintaining them.
The only caution I would issue is to do it slowly. If you edit/add answer to 10 or 15 new posts it might take a couple of days for them to go off the front page due to our relatively small volume of posts. However doing a couple at a time is a great way to keep old questions fresh and relevant.