The purpose of an AP is to extend coverage, e.g., provide coverage to areas where the wireless router doesn't reach.
Therefore APs have to be "far enough" apart. On the extreme if they were right next to each other you wouldn't get extended coverage.
They also have to be "close enough", i.e., some signal overlap, or the client will hit a dead spot (no connection) as he moves from AP to AP.
Therefore at least some APs will share airspace. I'm thinking APs that share the same airspace should use different channels or you will take a performance hit (akin to having a nearby neighbor's router using the same channel as you).
Now if your APs are lined up in a clear daisy chain fashion (A==>B==>C) it's entirely possible that A&C won't overlap and could use the same channel.