There isn't really a strong notion of "post order" except within the context of a single user. Between users, however, it there isn't a specific way of interpreting that.

The posts do have dates but I am hesitant to rely on those since someone could "fake" their dates to always get sorted at the top of the list. Of course, this may not be an issue since someone could just stop following someone who is actively harming the experience of their followers.

The other useful order is just the order we discover them by seeing updates from followed users.

Generally, the combined view on the main page starts with is initially populated just by a dump of data already known, grouped by user. This is probably the worst order but was the most basic, back when I just needed "some" kind of combined view.

Probably worth re-thinking this initial data order when the new UI overhaul is done for 4.1.