If you are parsing html or xml (with python), and looking for certain tags, it can hurt performance to lower or uppercase an entire document so that your comparisons are accurate. What percentage (estimated) of xml and html docs use any upper case characters in their tags?
Only if you're using XHTML as this is case sensitive, whereas HTML is not so you can ignore case differences. Test for the doctype before worrying about checking for case.
When matching html or xml tags, should one worry about casing?