May I use the Streamlined Sales Tax Registration System (SSTRS) even if I have a legal obligation to register in a state?
Answer:   Any seller that wants or needs to register in all the Streamlined full member or any of the contingent or associate member states may use the SSTRS.

States may require additional information if you have a legal obligation to collect a state's sales tax if you are not already registered in that state.  If a state needs additional information, they will contact you.  

The SSTRS has a link to state contact information.