If you do not run another instance of C::B, that does not allow DDE-/IPC (started with -nd/-ni), it does not matter.
But if you have a running instance of C::B, that does not run a DDE-/IPC-Server, it will fail, if "Allow only one running instance" is checked, because the signal can be send, but "nobody" will answer and no new instance can be started.
That's the normal situation for me, if I work in C::B on C::B.