When should I tell a potential employer about a spousal accommodation?
I don't believe there's a universal right answer for this question. I think it depends on the level of accommodation required, as well as the particulars of your own situation. In general, if you're not being considered for a tenure-track or permanent-level position, there often isn't a lot you can do in terms of getting spousal accommodations, unless you have a very generous employer. In that case, I wouldn't think there's ever a good time to bring up the matter, because there won't be much in the way of possibilities.

For a tenure-track or equivalent position, on the other hand, I think that I would wait at least until the level of having secured an interview before bringing up these issues. Any earlier, and some employers may (perhaps illegally, depending on jurisdiction) wash their hands of the issue by skipping you in making the list of finalists. Once you've gotten the interview, though, that's no longer a concern.

Whether to mention it during or after the interview depends on the ability of the school to do something about it. I would lean towards mentioning at the interview, and particularly so f the school has a "dual-career" office in place. If you wait, then you may spend your time negotiating and having to accept or decline the offer without a firm commitment of help in hand at the time.