If you have a TXPDR that is equiped for mode"c" you should use it. It is essential for TCAS to operate, it does not matter whether it is unverified or not. If you are operating under CAS and you enter it at the top of a loop for example and you dont show "C" and get in close proximity with a TCAS equiped jet, the first thing the pilot may know about you is seeing your face in his.