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?
XML (and XHTML) tags are case-sensitive ... so <this> and <tHis> would be different elements.
However a lot (rough estimate) of HTML (not XHTML) tags are random-case.
When matching html or xml tags, should one worry about casing?