ANDY'S ANSWER: The purpose of the scrotal sac is to let the testes hang away from the heat of the body because they need cooler temperatures to produce sperm. If the testes are cold, the scrotal sac will wrinkle up and pull the testes closer to the body. Other things may cause the scrotum to contract and push the testes up, like sexual excitement or even exercise. When the testes are warm, like in a hot shower, the muscles of the scrotal sac will relax and the testes will drop down. (This is a good time and place to do a monthly examination for any unusual lumps which could indicate testicular cancer)
Your scrotal sac should react automatically as climate control for your testes by pulling them up or letting them down as needed. If they are spending all of their time close to your body except for in the shower it could potentially cause a problem with fertility. If that is the case, it wouldn't hurt just to get things checked out with a doctor to ensure there isn't a problem.