From another long time TCAS user who has to use Class G every time we fly, please always use Mode C unless you suspect it is in error or ATC have asked you to deselect it.
With the help of our TCAS we very often see other aircraft that obviously have not seen us at all (I say that because many don't obey the rules of the air, with regard to rights of way - so we have to do the avoiding for them).
In the obvious interests of flight safety, we don't mind doing the avoiding as long as you help us in every way to do the seeing. Mode C is far better than plain vanilla mode A alone and helps us keep visually scanning the rest of the sky as well as the sector you appear to be occupying at "some altitude or other".
Ta.