Our boxers have all done their own thing when it comes to barking. Our first family female actually stopped a burglar from entering our house one night--we awoke to frantic fierce barking in the middle of the night, and could not get her back to bed. She was trying to get through the front door--and the next morning we found footprints outside in the bushes. She was exceptionally watchful, perhaps because my brother and I were small when we first got her. The high point of her day was when the mail came--she'd lauch herself at the slot, barking madly, and grab the mail as it came through. Each day was the same excitement--we never got a piece of mail without teethmarks!
Our second family female didn't bark at all until she was almost 3, and was surprised by a meter reader in the backyard. She yelped and ran one way while he yelled and ran the other. After that, she only barked at toddlers and people in motorcycle helmets. She was sweet, but not particularly fierce--she would stay upstairs hidden from sight when we weren't home, and only come out when we called her. Friendliest dog in the world, though!
The boxer my parents have now (2 years old) is a phenomenal watch dog and growls or issues a short, sharp bark when she hears any noise at all, be it a car in the driveway or a door slam in the house. I feel 100% safe with her. Georgie, my 7 month old, doesn't get too excited about anything and only barks at me when she wants a treat. She, too, greets all visitors with licks and wags of the tail. But being with my parents' dog seems to have taught her to be a bit more vigilant--she follows Mattie's lead, even though she's not quite sure what she's doing. So perhaps she'll turn out to be a bit more alert yet! But none of them have barked incessantly, or for no reason, which is how we like it!