Where you start depends entirely on just how wide the chimney breast is. What you don't want, as teacake says, is the overlap being less than about 4". You also do not want to leave yourself with less than about 6" either side of the face of the breast because as you wrap around the angle, you will find the paper pulling away from the butt join.
If your paper is patterned, it is also a good idea to ensure that the pattern across the top of the chimney breast matches the pattern either side of the chimney breast. So that when you look at the wall as a whole, it is symmetrical. Hope i've explained that so that you understand what i mean :-)