Can't be worse than not having the locale at all. Not to mention that there is an option --posix to assure compatibility to POSIX.1-2008, AFAICT. If there are incompatible changes, then its my job to worry about recreating the custom locales.
Since the postinst builds just a subset of all locales and since it even maintains a list about it, I would suggest to erase and rebuild only these locales. Thanx very much Harri