Every time CA Top Secret initializes it reads the TSS parmfile which contains all the configurable security setting for CA Top Secret including EXIT(xx).
So having it on in one system sharing the SYSPLEX doesnt turn it on for all. Unless of course all the systems share the same TSS parmfile. If they have separate TSS parmfiles, then it would only on for the systems that have EXIT(ON) in their individual TSS parmfiles.
Yes its OK to have TSSINSTX only running on one system.