I agree keep any emission part you take out just in case the laws where you are change or are enforced more greatly. every mechanic will do the inspection different and may miss or not check everything. some that are looking for ways to make the most $$ will check everything little they can fail you for. Also before going straight pipe check you states DVM web site for what the inspection requirements are. Mine are surprisingly strict considering that Diesels are exempt from emissions testing. I have to keep every emissions part mine came with, including the Cat, EGR, muffler and all. they won't check that it works but if its not there, looks tampered with or leaks I could be failed.
Here is Pennsylvania's inspection code for passenger cars and light trucks.
http://www.dmv.state.pa.us/pdotforms/pu ... tion_e.pdf