You could technically do either, however the further into flowering, the harder it becomes, atleast that is what I believe.
Additionally, you want to keep a mother for cloning, and this way, it is kept vegging, indefinitely. This way you have a source of females to grow at will.
The "best" time? I'd say about a week before you turn the lights back to 12/12. This will give the plants a week to recover from the "haircut" and give you some very healthy cuttings. You CAN take cuttings up to three weeks into flowering, but I wouldn't recommend that you take it past two weeks.
ok i was just curious. i dont want to clone anything from this grow, i think my next grow which will be purp and space queen, we will choose a mother. purp probably. just thought id be prepared