As a new tandem rider (my wife and I have been riding since July 2010), I have noticed that my back wheel is a magnet for riders. While this doesn't bother me if the rider lets me know he/she is on my wheel, I have found that most just hop on with no notification. My stoker is good about letting me know when someone is on my wheel unannounced.
Last week during the Shiner GASP many riders would hop on my wheel on downhills and on the flats (not many of those) then ride around on the climbs. I was generally OK with that and found most of the cyclists friendly and appreciative of the pull as we had a stiff head wind for most of the ride.
Where I became concerned was late in the ride where my stoker and I were tired as were the riders who would hop on our wheel. I know I was not piloting the bike as steady as earlier in the ride and did not feel comfortable having a rider close on my wheel where our combined fatigue was a recipe for trouble. When these riders would see the tandem, it was like they were pulled by a magnet to our wheel.
Is there a general consensus by the wealth of knowledge on this forum on how to handle this situation? I don't want to be rude to other riders. On the other hand, when we are tired, I just don't want the added responsibility of ensuring the safety of someone on my wheel, let alone our safety.