Hi Andrew. In my opinion, ABSOLUTELY you should have, and use a liability waiver. Potential liability extends to your training activity whether it’s for free or you are charging for it. Even non-profit and charitable organizations use liability waivers for many of their public events.
General rule of thumb – when in doubt, use your waiver! The worse that could happen if you use your waiver and never need it is that you have an extra piece of paper to file 🙂