"When should you rebuild an index" by Jonathan Lewis:
http://www.dbazine.com/jlewis14.shtml
Conclusion

There is only one sound argument for rebuilding an index:

Will the total cost of rebuilding the index be a reasonable price to pay for the resulting benefit to the system?

The answer to this question is frequently a resounding NO. In fact, sometimes the overall impact of rebuilding an active index will be detrimental to the system. However, there are still plenty of misconceptions about indexes that result in DBAs the world over wasting valuable time and effort rebuilding indexes unnecessarily.

BUT, if you have a regular window when the system is not in use, have a simple batch job that runs in that window without putting pressure on other batch tasks, and that batch job can never fail — then you might as well rebuild all the “safe” indexes you want to; there is often a slight performance gain to be had, and if it costs you nothing, then you might as well take it.