The etiquette advice we received on the first day of class is not meant to be all-inclusive. For instance, it did not include anything about ball management, and we all know how important ball management is to a smooth game. If you need things spelled out in detail, let us add a point #5 to this list.
5. Extra balls should be kept in a pocket or ball clip, in your hand, or against the fence. In other words, it is not okay to regularly keep an extra ball in the sweaty bosom of your tank top. Especially when the ball belongs to someone else. For one thing, it looks ridiculous. For another thing, yuck.