Is it safe to use WordPress generated classes?

Post ID Specific Classes are not reliable at all. The ID is based on when the page was created ( in what order since IDs auto-increment ). If the user “accidently” permanently deletes the page and needs to recreate it the page will have a new ID and thus not use any of the predefined styles. The best bet would be to use a Custom Page Template so the user can assign it to a specific page. This is probably the most user-friendly method.