Would it be safe for me to completely empty and re-index the contentwords table?
Yes. But note there is no need to empty it. The re-indexation process already takes care of it.
Do I have to do this because it exceeds 500 mb?
No. Re-indexing it every 6 months or so should be OK. This depends on site activity, site age, and some other factors. The growth of the table should look like a logarithmic function.
