an informed consent should be the least a client signs, whether or not you have professional liability insurance. It should be part of the contract you have with the client, together with health history, the contractual terms and a physician’s release form as applicable.
I teach sometimes small group exercise with no contractual commitment from the participants, and the one form I have everybody sign is a liability waiver.
That would be a big “yes” from me. I actually have people sign both an informed consent & a release of liability waiver form. You can never “CYA” enough!