I find not closing my <li>s and many of my table tags (e.g. https://gist.github.com/1117438) lends to much more readable and maintainable markup. <head> and <body> I'd leave explicitly there, the tag autogeneration rules conflict with HTML5 elements in oldIE.
IMO, you shouldn't ever drop optional tags for a byte payload reason, this is what minifiers are for (htmlcompressor is extremely robust). But sometimes it makes sense to drop some closing tags so your source isn't an overgrown garden of angle brackets.