1. Obviously, whatever we see and hear on the series must count.
2. But there is a second category of what we see and hear, which warrants a little more discussion: screen caps, freeze frames, and the sorts of things we may see not via conventional viewing. Consider, for example, the newspaper clipping that brings Jack to the funeral home in the finale, and perhaps to the point of suicide. Nothing in that clipping was really viewable in real time. Had the producers wanted us to see (or hear) info from this clipping in real time, they could have had Jack talk about it, or given us a close-up of the clipping that stayed on the screen long enough for us to read it. Instead, they gave us a shot which, through enhancement after the broadcast, enabled us to see some information (including the April 5, 2007 date, as later verified by the Los Angeles Times). I'm not saying any of this information is unreliable or unworthy - only that we should be aware that it occupies a different niche in the narrative, if only because the producers chose to not show it to us directly.
3. But the newspaper clipping brings up another category of evidence: reports of what the clipping or anything else says, based on connections the person giving us such a report has to members of the crew, actors, etc. Although these reports might be interesting - I've seen a least one complete rendition of the newspaper clipping on Lostpedia - I don't think we can take them as decisive. Even if the reporter is telling the truth about how this info was acquired, we have no way of knowing if the crew member or actor had accurate information in the first place, was telling our reporter the truth, etc.
4. While we're on the subject of information provided by crew, actors, producers - how much weight should we give to either leaks or explicit info provided in publicly accessible interviews? I would say we should not take this information as decisive, either - although the producer is not likely to be misinformed, he and she might be deliberately misleading us - providing misdirection - so as not to spoil a surprise. Disinformation has always been alive and well in the creative arts.
So, all in all, I would go with the great literary critic I. A. Richards, who way back in the 1920s said the only reliable evidence is the text itself - or, in the case of Lost, what we see with our own eyes on the screen, or via a screen we know to be 100% reliable....
Lost New Questions: 1. How Far in the Future? ... 2. Who's In the Coffin? ... 3. Who's Waiting for Kate? ... 4. Who Is Naomi's Boss? ... 5. Is Mikhail Immortal? ... 7. Are the Flashforwards Desmond's Flashes?
Lost Season 3 Finale ... Flashforwards
And see also Lost's Back Full Paradoxical Blast: Season 4 Episode 1
Lost: Keys to What's Really Going On
The Plot to Save Socrates
"a Da Vinci-esque thriller" - New York Daily News
"Sierra Waters is sexy as hell" - curled up with a good book