The robot will very likely hit my frontpage anyways, so I'd rather have them look (and not find) for a meta tag there than to produce an additional 404 error which will, depending on the content of said page, waste a considerable amount of bandwidth.

Besides, my frontpage is either heavily fragment- or just page-cached anyways - especially for anonymous user - so it can be served directly from RAM for all intents and purposes, so a hit to the frontpage is really, really cheap.