The general thinking on indexes would seem to be 'Rebuild it regularly' if you want it to perform well, but that is not necessarily the correct way of thinking. By rebuilding you use-up a lot of system performance and down grade your systems performance during the rebuild as the index will not be available. This article is somewhat dated but neatly goes into the pros and cons of regular rebuilds.
We run a Data Warehouse application which has some very large indexes and we don’t regularly rebuild them, although we could probably find a recurring 8 hour time slot some time during the week.
