Most auto-focus cameras come with an audible tone or beep to indicate when a shot is in focus. Wedding photographers will probably tell you that they have the beep function disabled -- with good reason -- you don't want to be the one with the noisy digital device during a quiet wedding ceremony. However, my general rule of thumb for the auto-focus beep is -- use it when you are doing kid photography -- especially when you may need to move out from behind the camera to engage the child you are photographing.

Funny story -- recently I was at a children's photography workshop with over a dozen photographers in attendance. When it came time to shoot, there were so many beeps in the studio it sounded like a family of birds had been let loose inside!