Do you need an inspector for the septic system? Yes and No.
Yes if you really want that particular house and want to leverage your ability to make the seller pay for any necessary repairs.
No if you just want to wait for the inspection from the OSSF - Onsite Sanitary Sewage facility - licensing authority. For instance, in Lakeway, if you are selling your house, the Lower Colorado River Authority - LCRA - inspects the septic tank and system before the license is passed from the seller to you. Consequently, if there is a problem, it will materialize then. But what if this occurs after your option period? You can still make demands but you no longer have the leverage to just opt out. Hence, if you think it is worth it, get an inspection of the septic. Just pay the septic inspector what they charge so you can sleep better and retain your leverage.