I thought it was strictly tabboo and forbidden by most airlines these days for obvious reasons, and that crews risk their jobs to do so. Usually, I have been on the jump seat from take-off and for most of the trip coming from my country to the UK, but only because of being a close friend of the Cpt or FO, or if the friend FO was not on board, would contact the Cpt of the flight saying that friend so-and-so is on board, if he can accomodate him. (the good thing about a small airline where most crews actually know each other or are friends) Even so, I am always obliged to exit the cockpit once over UK airspace and for landing because of UK regulations. I know this, and the crew friends remind me of this. On TO from UK, I have sit in cabin, and get called up after leaving UK airspace.