I think I recall Paul Irish once saying even closing tags of certain types (<li>, <p> etc) were also unnecessary. The start of the next tag should also signal the close of the previous one (unless you want to nest stuff) -- so whether the </p> or </li> tag or whatever is there or not makes no difference.

HTML parsers cope quite well with that (and have done for a while) but I don't do it. Although I am guilty of not bothering with the type declaration in scripts that are JavaScript <script src="..." />

I know I'm just plain lazy and wrong.