there are specs for knotching and where and how big. Most knotching around here is dead center of the rafter, joists or whatever. Its the structural integrity that matters. A hole in the center is less likely to take the load bearing capacity away from the timber as a knotch on the side.
As Dodgeman so elequoently stated, " its all in using good judgement". People do feel that oh well it will be covered up and hidden so I can do as I want. Its not my house.
When I tear into a home for some repair. I can either cultivate a good positive bond with the builder, electrician, plumber as I do my repairs or I can say what a idiot. When I do electrical work and the previous electrician left a little extra wire or a wide loop, its so much nicer than one that stretched the wire soo tight that the connections cannot be re attached. Its just pride in your work and respect for the next person opening up this area.