None are held back, and you can't refuse to accept the online "letter group" that you get; it isn't assigned until you are checked in, and once it is, it is permanent. If you are eligible to preboard you get to ignore it, but it doesn't really matter for others; you get out of your group and everyone else moves back one place in the "virtual" queue.
When SWA first started online checkin they distributed the groups and held some back, but they don't do it anymore.