MY EXPERIENCE:
with DFK, you have to initiate. If they don't like it, they'll do the turn the head thing. As to BBBJ, I've usually had to ask, although RARELY some of them have just gone down on me w/out having to ask. Sometimes I offer extra for it. If they agree, great. If they disagree... I'll usually finish the session, but it usually indicates it'll be the last session. CBJ... I dunno... To paraphrase a comedian enjoying a CBJ is like enjoying a steak with a balloon wrapped around your tongue.