There is a current thread on the IETF AVT e-mail reflector where
talk is of making RTCP mandatory (i.e.-changing it from a SHOULD
to a MUST in the RTP specification).  The last I heard was that the
MUST would apply to implementing RTCP, and not necessarily to
using it.  Presumably the application level call signaling protocol can
be responsible for signaling whether RTCP was to be used.
 
   If an endpoint implemented RTCP it should at least benignly ignore
received RTCP reports and would not need to send any.  So a given
operator could specify that RTCP not be signaled if it were not
practical in their network.  Obvious examples would be wireless
networks and cable networks where there are access bandwidth
constraints.

Cheers,
Rex