Where do you draw the line?

I mean, ASIWYFA is a no brainer, ain't nobody got time for And So I Watch You Fro... ZZZZZZZZ
and
at the opposite end, you can't just say R and have people know you mean radiohead (can you?)
but

Where's the line?

Two letters? Three letters? Four letters you prick?

potential subthread ideas: Which bands is it acceptable to say the full name of? "Do you like Rapid Eye Movement?" "who?" "REM" "twat".
What does The XX stand for? The Xenophobic Xylophones?