When travelling somewhere, I always take a set of tickets with me, just in case I have to go home via an alternative routing (I check out possibilites way ahead of my holiday). Free seats are given away regarding a priority list that takes into account your status. A CEO of our airline has a higher priority than myself, I have a higher priority than my girlfriend and she still has a higher priority than any external staff travelling. This means that it is highly unlikely to be put on a nearly full flight, if you have staff of that airline or even airline alliance travelling at the same time.
Jump seats is a good point. Our company does actually not allow staff of other airlines to travel in the cockpit, but final decision rests with the commander...