In some instances service agreements are mandatory under the rules of the NDIS. These include: specialist disability accomodation, the claiming of an establishment fee, negotiations of cancellations.
However many services do not require one, for example: carers, occupational therapists, personal trainers. It is up to the participant as to what they think will best suit the needs of their family when this is the case. Often fixing a price for a period of times and establishing you rights can be beneficial.