My professor used to say, include call-outs even at places where there really doesn't need explanation, because call-outs make you sketch look more intelligent than it is, and it's not like anyone's really going to read them.
To me, everything on the sketch page is a graphic element, like arrows and call-outs. Therefore, I try to be strategic about where I place them to create a good composition flow. Of course, this will also mean you have to plan your sketches so that you leave the right spaces for your call outs.
It's not the cows' fault, stop molesting them!!!