They are...only place folks put them (at least that general area). I haven't added them...I want to see how dirty Morg's pipes get if he ever drives in the rain.
Rolling the metal or adding some wire for the header exit hole can't hurt, but after several years, there hasn't been any cracking without either on mine (nor have I heard of others having such a problem).
Doing a nice rolling job can be tough given the angle cut towards the rear of each side...I've seen some bad ones, and then folks end up cutting the metal more to get rid of the rolls and you end up with overly large holes. Practice on some sheet arruminum!