The way I'd picture it would be that clients would keep the old behavior, and the defaults would just change for the web interface. I would say that we could encourage client maintainers to switch to the new system, but that presupposes the existence of client maintainers. :P

But you have a point about the new behavior--now that I think about it, it's possible that even with normally configured systems the posting page defaults to posting with the time the post started rather than the time that the post was actually submitted, and somebody might like that. So if we were to change it, we'd have to warn people in advance.

I still hold that that's a terrible default behavior and should never have been implemented in the first place, though.