The other thing to consider is that, within the game itself, there seems to be a slightly greater emphasis upon the NCR in terms of questlines available the player, etc. It's not unlike Lucasfilm's policy concerning multi-ending Star Wars video games, in that the "Light Side" (or rather, Republic/Rebellion endings) are considered the de facto conclusions for canon purposes. Not saying that it'll necessarily turn out that same way here, of course, but simply at a glance, again, the storyline scales are slightly tilted in the NCR's direction overall.
Usually I'd agree with you, but I don't for three reasons:
1) One could argue Independent is just as "good," if not more.
2. NPCs from all across the Mojave imply both NCR and Legion need to lose. Graham disses the NCR, Ulysses disses the NCR, Cass, Raul, Arcade, Julie Farkas, etc etc etc. It seems odd for two main characters from DLCs (Graham and Ulysses) to imply the NCR is doomed to fail, then for them to win.
3. The Courier is our first neutral karma hero. No one contests the defeat of the Master's Army as being a good thing, no one contests the defeat of the Enclave as being good, no one contests.....the repeat of those two in FO3 as being good. Whoever wins in the Mojave though? Someone has to lose. It's impossible to please everyone here, cause no matter who you choose, a group or a person is going to be affected negatively. It's also impossible for the Courier to not get blood on his hands: you HAVE to gain bad karma at one point in the game and you HAVE to kill and/or totally doom something or someone. For this reason, I think it's a moot point that the good ending must be canon, because while past actions of past protagonists have implied a good ending, the Courier seems to be a rather neutral protagonist, doing whatever he likes.
Since we already know a "pure good" ending is impossible, then why settle for a "75% good?" They'll probably just pick whichever is most interesting, and the most interesting choice seems to be House or independent. Independent means they can go in any direction they want, House means that both the NCR and Legion survive to see another day, but both will walk away with internal conflicts. This means that we now have THREE superpowers (Great Khans may likely provide us with a fourth, hope they do) in the area and they're free to write the next conflict however they like. Legion could improve itself morally, NCR could become chaotic; anything's possible.
I personally hope House wins but wouldn't be surprised nor disappointed if Independent was canon.