The polite and mature thing to do in any party is to:
1) See what it's for. Usually they'll say "practice" at "whatever stage" or clear or farm.
2) Judge your own skill by how well you know those phases.

That's it. I think it is completely acceptable to go into a clear party if you've successfully gotten to enrage several times AND been close enough to clearing that with just a tad more DPS you could do it. There are lots of players who clear it once then think they can farm, and I think that's where a lot of issues happen. You're very likely NOT going to be perfect at it on your second clear try.