Back when Freddy vs. Jason unexpectedly screened in advance for press, rumors were swirling that the last five minutes would be omitted, keeping it a secret who wins so that readers couldn't possibly be spoiled by condescending critics who wished to ruin it for them, and critics who really wanted to know would have to pay to see it. Thankfully the talk turned out to be cheap, and we all saw Jason emerging from the water with Freddy's severed head.
What if we hadn't? Would that have been fair?
For the sake of our argument, let us dispense with the Kevin Smith sentiment that reviewers should pay to see everything. That's a different conversation. Presuming the status quo, that reviewers do in fact see movies in advance - and advancing the proposition that we do, in fact, "pay" by taking a couple of hours out of our day to work hard on a review, which is what we do for a living - are we entitled to see the version of the movie that will be in theaters?
You'd think the obvious answer is, "Yes, of course reviewers should review the same movie that audiences will actually be seeing." But heaven forbid we complain that a post-credits moment in a Marvel movie is withheld from us. Then we're told those scenes aren't relevant to what we're paid to do in assessing the themes of the film, and are a special treat for paying fans only. Because none of us critics can be fans, of course. That part of the argument is like when Comic-Con vendors refuse to sell exclusives to anybody with an exhibitor badge, though of course most exhibitors are fans too (that, or masochists). Newsflash: most movie criticism online is done specifically by FAN sites.
So the question becomes, then, are those scenes relevant to assessing the movie as a whole?
On one end of the spectrum, you have Napoleon Dynamite, with a post-credits scene that essentially functions as a mini-sequel and, I would argue, changes the film thematically by showing that the awkward romantic elements played earlier for cheap laughs actually do pay off. That said, this scene was added weeks into the film's release, so initial audiences had no better shot at seeing it than critics did - but future assessments will have to take it into account. (As Napoleon Dynamite debuted at festivals, we should also note that films sometimes change between festival showings and full-on release, but reviewers usually note that.)
On the other end, you have the blooper reel, common to Jackie Chan films and often spoofed by Pixar. These are definitely not part of the movie proper, but a look at how it was made.
Where do Marvel's post-credits scenes fit? I think we can strongly assert that their mid-credits scenes are often essential: knowing that Thanos sent the Chitauri is an important plot point, as is knowing where the Winter Soldier is at the end of Captain America 2, or that Thor comes back for Jane. But do you need to know that Deadpool is alive at the end of X-Men Origins: Wolverine? Or that an Avengers trailer capped off the first Cap movie?
I'll say this, from firsthand experience: it would help. With Guardians of the Galaxy, rumors were swirling about a certain plot point, and I opened my review by confirming, or so I thought, that said plot point was not in the film. I later saw bootleg footage online of the post-credits scene being held just for the paying fans, and, well, I looked the fool. Similarly, in the Village Voice review of the first Captain America, Karina Longworth wrote of the post-credits scene: "(Spoiler alert: Captain America doesn't have one)". This was true of our screening; it was rendered untrue by the release print. Ha-ha, stupid critics, right? Should have done our homework? Shouldn't have even remotely speculated about things we didn't see? Heaven forbid someone paid to analyze something talks about larger implications.