The major, high-level events are canon, some details are not. The existence of the Midwestern Brotherhood is acknowledged in Fallout 3.
I took that as more of a "shout out" to tactics than any real statement of cannon. I would agree we can take tactics as cannon in the areas it isn't contradicted, I suppose, but I would expect Bethesda cannon is FO1, 2 and 3 only, personally.
Actually, I bet Bethesda feels FO3 is essential cannon and even FO1 and 2 can be modified or partially exempted, if the need arises.