I also just know it, even with the same range (‘P’–‘U’) of uncertainty. (And yes, I'd composed enough of this reply before reading the parent comment that this is an independent datum.) I sometimes even mix up ‘R’ and ‘S’ after thinking about it. (I never go back to the song, although I certainly do know that too.) I have been known to touch-type the alphabet in order when checking out a new font.

This has been useful to me. As a teacher, I alphabetise papers before recording grades, and it's handy to be able to do this quickly and correctly. (I'm pretty sure that I just knew it before I started regularly using that knowledge, however.)