Personally, I’ve always disliked uppercase tags in the code. Uppercase characters in digital format is often perceived as screaming, and if that’s true, boy, have I’ve been screamed at by a lot of code I’ve seen. It looks bulky and feels like working with skyscrapers when doing a cut-and-paste operation, one expects the computer to start screaming from the effort.
Estethics aside, there’s also a good technical reason for not using uppercase tags: it’s not allowed in any flavor of XHTML. To quote the XHTML 1.0 specification:
XHTML documents must use lower case for all HTML element and attribute names
As a follow-up to this, if your XHTML/HTML is indeed lowercase, make sure that your tag-specific rules in your CSS is lowercase too. Otherwise, you might not get the behavior you expect (you will not get it to work with lowercase XHTML tags and uppercase references in your CSS, if it’s sent with the MIME type
So, if you like and use uppercase tags, please lowercase your darlings. For me, and for the future.