I've been omitting optional tags for a while.

One thing I've noticed is that bing webmaster tools will report "The title is missing in the head section of the page" when there is a title, but no <head>. Maybe bing can't properly crawl pages without a <head>. Another service I've used had the same problem, but can't remember which.

So it might be worth being careful with omitting <head> - and maybe other tags, I'm reconsidering whether it's a good idea.