Answer: You must remain registered with all of the Streamlined full member states or cancel your registration with all of the member states. If you only want to register with a few states, you should not use the SSTRS.
When registering you have the option to indicate if you currently have no sales and do not anticipate sales in a state. If you check that box for a state, the state will not require returns until you have sales in that state. If you did not check that box when you registered and you have not had and do not anticipate sales in a state, contact that state directly to determine if you need to file returns with that state.