Do I need business liability insurance or can I have people sign a liability disclaimer?
Liability disclaimers and waivers are not always iron-clad. In addition, under some states' laws, you cannot ask someone else to waive your liability. So that you don't get caught short in either of those two scenarios, it's strongly advisable to have insurance in place to cover those kinds of claims -- in addition to the waiver.
Another benefit of the insurance is that some policies will also cover the defense costs...which could otherwise drain your operating cash.