Sometimes you also should consider the OOC circumstances. For example I consider it at least a matter of proper MU* etiquette to ask in a page before I actually join a scene unless it's in a really public place; that is, I won't really check first to go to Elysium but if it's something happening at bar #5 on the grid and it's just two people then I will. People might not want others there which is fine with me; if they're RPing a quiet dinner date then anything I do to engage them would be disruptive unless that's what they're looking for.
Then again it's been known to happen that you'll ask if you can join something already going, get there, and figure out there just isn't anything for you to do. Maybe those involved say they are (or actually are) OOC okay with others coming in but they are RPing having dinner and any attempts to hook yourself in that or to engage them go unanswered. That's a bit more awkward since you now have to consider a reason to extract yourself from that situation unless you know them well enough to be sure actually pushing the matter IC won't annoying anyone present.
Frankly I'd much rather be told ('just so you know, it's probably going to be boring for you if you come') than spend half an hour finding this isn't the scene I was looking for.
Edit:
@Rook said in Good writin'.:
@Arkandel
I dunno, Ark. I could really go with that, since it was entirely IC, and the mob was right outside. That implies, to me, that it is a currently-running, happening-right-now plot, and it is entirely plausible that someone would burst into a room all bloody and needing help.
There was no mob outside before we got there, and (as I remember the circumstances at least, this was a while ago) no metaplot about that sort of thing happening in general.
But basically the "I walk into the room bloody, bruised and half dead, someone save me!" has been an attention-seeking trope for a long time. 
Sorry to nitpick your example. 
Sokay!