Who it's for, and when
The 69 is a universal position: it suits a straight couple just as well as a gay or lesbian one. The side-lying version is the most restful of all. It works well for partners of different builds, since nobody gets squashed, and for anyone who wants to take their time.
As for timing, it makes the perfect starter: you both work up an appetite before the main course. And if the amuse-bouche was a generous one, the course that follows can be lighter. The 69 can also be a meal in itself, because making love isn't only about intercourse.
Precautions: if there's any pain, irritation or infection, talk to a doctor or a sex therapist. If you're pregnant, have a painful back or neck, or limited mobility, ask a health professional for advice first. During pregnancy, never blow into the vagina: air can get into the bloodstream (an air embolism), a rare but serious accident. And with a casual partner, protect yourselves: a condom for oral sex on him, a dental dam (a square of latex laid over the vulva) for oral sex on her.