Apologies, I should have included a reference to FS#49104 [1]. As you both have suggested, it is used in some locales, but more generally, by adherents to the SI [2] and to the ISO 31-0 [4], which include the scientific and engineering communities even in English speaking countries. It is possibly a good idea to insert this space conditionally if the locale in use specifies adherence to the SI; however, since there doesn't seem to be a universally followed rule regarding this topic [3], it may be also be a good idea to default to a well-defined and widely-followed norm where this is regulated, such as the SI brochure and ISO 31-0. Let me know what you think is best.
[1]: https://bugs.archlinux.org/index.php?do=details&action=details.addvote&task_id=49104 [2]: https://en.wikipedia.org/wiki/International_System_of_Units [3]: https://english.stackexchange.com/questions/3281/should-there-be-a-space-before-a-percent-sign
