I'm a "loner" when it comes to travel; I rarely travel with others from my company. I can think of four segments where I did -- we were both in coach on one, both upgraded to first class on two others. On the fourth segment, I upgraded while my colleague sat in coach, but it was no big deal, since we were peers.
But this thread reminds me of something that came up on one of my very first business trips. I was AA Gold at the time, but I was headed to Fargo, where AA doesn't fly, so I ended up on NW, and signed up for their FF program at the airport on my outbound trip. When I checked in at the kiosk for my return trip, the kiosk spit out BPs with very low row numbers that said FIRST on them. I was very certain that I had booked coach seats, so I was very confused. While I was sitting at the gate, staring at my BPs and trying to figure out what was going on, I was shocked to see a couple of execs from the customer I was working with. They were from a different office, and they were also heading home on my flight, so they came up to say hello. Now I was totally confused as to what I should do. It turned out that my seat was indeed in first class, so that's where I was sitting when the execs walked past. I told them that I had no idea how I had ended up in first (which was the truth), but they just laughed and said I had done great work for them, so I deserved it.
At the time, I was new to the whole frequent flyer game, I didn't know that I'd get a first class seat until I showed up at the airport, and I didn't know that my customer would be on the flight until a few minutes before boarding -- so I was very poorly equipped to handle the situation. Today, if I knew I'd be flying with a customer (or my boss, for that matter), I would call the airline ahead of time to cancel my upgrade request. It's not worth any potential ramifications, just to sit in crappy domestic first (I'm a 100% domestic traveler) for an hour or two.