I didn't know you had to - Maybe that's why I never have any volunteer stokers!
Robin and I were descending a hill somewhere in Scotland once, both tucked down. As we approached a point where I might need to steer and not be travelling at 45mph, I sat up and immediately, with the unbending of the digestive system, let fly _fortissimo_.
I inferred Robin's head was still near my backside when he burst into helpless giggles.
At that point I burst into helpless giggles.
Shortly after that we had to stop. You cannot hold a tandem steady if both captain and stoker are wetting themselves laughing.