Use release disambiguation not release-group disambiguation.
I found 2 examples why I would prefer the release disambiguation:
  http://musicbrainz.org/release-group/d3f057b4-7e89-3018-9f84-9835ed9b4732
  http://musicbrainz.org/release-group/6792b6d1-4e65-3c3c-9d20-d08aa1dcfc60
I did not find an example where the release-group disambiguation would be important (of course, that doesn't prove anything ;))