DAV:getlastmodified of collections

AFAIK, according to RFC2518, Section 13.7, the DAV:getlastmodified property
is not required to be defined for collections, but a server MAY define it.
Is that correct?

Then, probably the bindings of the collection are to be considered part of
the collection's state and it would make sense to set DAV:getlastmodified
whenever the bindings change:
- BIND, UNBIND (the coll identified by the req-URI)
- REBIND (the coll identified by the req-URI AND the 2nd involved coll)
- PUT returning 201 (the containing collection)
- MKCOL (the containing collection)
- DELETE (the containing collection)
- LOCK creating a lock-null resource (the containing collection)
- VERSION-CONTROL on existing version (the containing workspace)

Sorry for such a basic question, but I didn't find it in the specs.

Regards,
Peter