"Technically" sub-affiliations are optional.
Of course, as soon as the rulebook says that, it immediately goes into about how you'd be an idiot not to take them, because the Affiliation cost is dependent on the sub-affiliations, and you'd just be wasting points (in some cases, alot, the Home Worlds affiliation costs 50 points, but without the sub affiliations you gain -150 in traits) if you didn't take them.
So technically, but..yeah. You should probably go with one :)