in Class G airspace, ATC radar in most cases won't illuminate your
transponder, so it won't reply anyway, so as far as ATC is concerned it
probably doesn't matter if you have it on or not. However, TCAS
equipped aircraft will interrogate your transponder, so to be visible to
them you might want to keep it on mode3/A so that it doesn't report a
misleading altitude but does return a reply.