It crashes once every week or two for me so I chose this step and haven't chosen to rebuild it. It apparently re-populates itself over time. So yeah this IS safe to empty and be sure to empty it regularly, depending on your forum activity.
Code:
Click Here to empty the search index.
Apparently this is for when you click to "find all posts" OR "find all threads" for a user, you decide if you want it rebuilt or not.