[_mhp_]

Section 2 contains:

  A PATCH request can be issued in such a way as to be
  idempotent, which also helps prevent bad outcomes [...]
  Clients using this kind of patch application SHOULD use
  a conditional request [...]  For example, the client
  can use a strong ETag [RFC2616] in an If-Match header on
  the PATCH request.

So you can, but aren't obliged to.