'Course the milder alternative to that is, keep a pretty vague worldview ("You can be from anywhere on Oerth" rather than "If you want to be a wood elf in Middle-Earth you are by definion _________"); talk to the players; and work out as much of how you can still get them playing exactly what they want.

The biggest concern I figure is; having characters be at a similar tech level (and magic or ability scores counts as tech for this discussion). After that any GM preference winds up being thematic (for instance, you want everyone in your WtA group to be Garou rather than being this travelling menagerie of were-aardwolves and were-koalas with a single werewolf).