Just to throw in my 2¢, I personally strongly prefer the less-obnoxious
all-lowercase variant for all ‘#+’ keywords, though I do sometimes use
uppercase for things like #+TITLE or #+AUTHOR every now and again.

I think it would be nice to provide a defcusom to allow choosing between
upper vs. lowercase.  As of now, you should be able to get all uppercase
by customizing the `org-tempo-keywords-alist' list and by redefining
`org-tempo-add-block' (for the “begin” and “end” parts) in org-tempo.el.