Yes. it is a good idea.
Below is the recommendation from ACSMs Resources for the Group Exercise Instructors.
This segment of the manual was authored by Shirley Archer, JD,MA
“In today’s litigious environment, for the most protection, a GEI should carry professional liability insurance ($2 million per occurrence is the recommended amount), even when working in a business as an employee, where the GEI may be covered under the business owner’s policy. The reason for this is that it is not unusual for a single claim to result in a million-dollar judgment. Purchasing the best protection enables you to perform your job responsibly and to feel confident your career will not be destroyed by one mishap. Most insurers of GEIs provide coverage for certified professionals.”
You can purchase insurance through IDEA.
Editor, ACSMs Resources for the Group Exercise Instructor