Technically, I would agree. In other case, I would disagree as far as which event would actually hold alot of water into the future. But as far as I concern, its usually the the main event that matters (good guy path as a added bonus) with a few tiny reference of what happens in the past. Everything else is practically subjective to how the event went as far as how the player interaction to these sidequest and DLC. These I would see that have a high potential of not existing at all.
That's pretty much my understanding. "Canon" only ever really matter in a videogame to the extent that it's needed for the sequel to make sense. For example, it's "canon" that in Fallout 1 your PC didn't go beserk and kill everyone in Shady Sands; and is probably "canon" that you went through quest to save Tandi from the Raiders. (Since in Fallout 2, the NCR is said to have been founded by Tandi after she inherited leadership of Shady Sands.)
Generally, I figure anything that's specifically mentioned in a later game of the events in a previous one - or that takes place in supporting media such as the Fallout Bible; or if there were to be official movies, novels, or comics based on the Fallout Universe - is canon. Everything else that isn't specifically mentioned, or didn't have to take place for events within the games to make sense, resides in the same realm of probability as Schroedinger's cats...
For example, nothing in Fallout 3 can technically be considered "canon" at this point. (At least beyond the main events that will have to happen regardless of what you did in the game...) I don't say that because "I don't like Bethesda, or anything they've done with the franchise" (which would be untrue.) Rather, it's simply because there's no Fallout 4, yet. We don't, as yet, know what needs to have happened in Fallout 3 (or what events from Fallout 3 get referred to in that game,) for that game to make sense. If in Fallout 4 my character goes to Megaton and learns of some Vault Dweller who saved the town years ago, then it will be "canon" that the PC in Fallout 3 didn't end up blowing up the city.
Or for Mothership Zeta, for instance - if in Fallout 4 I run into a group of NPCs who say "man, wasn't it wierd when that alien ship abducted the Vault Dweller?" then it will be canon. If there is no mention of it, then it's not canon.
What should also be mentioned is that "non-canon" status doesn't necessarily mean that it never existed. Simply that, for a videogame where the events are largely subjective and designed to be (at least slightly) different with each playthrough, I find it rather a silly concept to try and delineate each and every thing in every game with a fine-toothed comb. Anything not clearly discussed within the following game, or that needs to have happened for the games as a whole to make sense, "could have" happened.
It's largely up to the player. We're not talking about "canon" as a holy tome where everything has to make absolute sense as the immutable word of God, here. We're trying to apply a word that was originally meant as such, to a videogame where the player has always decided what happens in the game.
Edit: By my rationale, there's only a handful of events in the entirety of the Fallout franchise that need be considered as "canon." Those are largely a couple of major plot points from Fallout 1, because they have to take place for the setup of Fallout 2 to fit in. Very little in Fallout 2 would necessarily be canon, then, because Fallout 3 makes very little mention of any of those events. In fact, beyong
showing up in Fallout 3 - making it "canon" that your PC in Fallout 2 had some encounter with him, and didn't kill him, I can't recollect anything at all that needs to have happened in that game for Fallout 3 to make sense.
In short, "canon" isn't about value judgements. It's simply what needs to have happened for the series as a whole to make cohesive sense, and have some unifying consistency. It's like Legacy of Kain, where in the first game you can choose the evil or good ending, but the sequel assumes you chose the evil one - making the evil ending "canon" within that franchise.