I plan to fly SFO-SEA-JNU on AS this fall. If I book saver (X), I understand I can only select seat after check-in. Is that during online check-in, or only at the airport? (But if I have done online check-in in advance, why would I need to check in again at the airport if I only have a carry on bag?)

Also, on the fare comparison chart, there is kind of a warning saying that saver fare ticket holders board last, limiting choice of overhead bin. In practice, is that such a big risk? If there is no overhead bin space, don't they just check my carry on bag without a fee? Thanks.