I prefer a z-bend on the servo arm.

I've had clevices and quick links, and sometimes the sides of the links can actually bind against the servo arm when the servo is at the extreme end of its travel.

As for wallowing out the servo arm hole.. I have to agree with others, a clevise, a quick link, will also do so. A ball joint won't.

As for precision... I don't know. From servo arm to control surface.. there are many different places where precision can be lost by a little slop here and there. And let's not forget about the simple possibility of flexing. Not only the flexing of the mechanical parts/pieces, but of the airframe itself.