My understanding was that even admins should tag, if there's anything there that's at all subjective (anything much above page-blanked-by-author, random text, whatever). It certainly seems more sensible that way.
(I usually try to, but...)
[[WP:CSD]]:
"The "Speedy deletion" policy governs limited cases where Wikipedia administrators may delete Wikipedia pages or media "on sight" without further debate. Non-admins can request deletion of such a page by adding an appropriate template (see below). The word "speedy" in this context refers to the simple decision-making process, not the length of time since the article was created."
I interpret that as admins being able to delete without tagging. The whole point of CSD is that it's for uncontroversial deletions. If there is anything significantly subjective about it, it should go through PROD or AfD.