Ignore everything the fear-mongers have to say: In the end it’s only up to you.
If you resonate with the idea and it makes you feel more comfortable, then the answer is absolutely yes (You can download a template disclaimer form here (In microsoft .doc format: http://lou.pm/ddoc). This form is offered as a service only and has not been reviewed by a lawyer. I do not guarantee its legal validity.) They have value as a form of prevention, in that they reinforce the message that people are responsible for their own health. From a legal viewpoint, however, they offer no real protection. In the USA a non-disclaimer form must be read out loud and signed in front of a witness to be valid in Court, and even that is not a guarantee you will win a lawsuit.
Just for the record, several laughter practitioners and Community Laughter Clubs have been taken to court over the years in various countries around the world, but none for health reasons so far. The consistent complaint is noise disturbance, and the request of the attacking party is to “stop the cacophony.” Read a typical example here.