IMHO, no slop anywhere in the control system.  And that is pretty much automatic with ball links.  In fact I can't think of any safe way to introduce slop into a control system with ball links.  

Some use play in elevator control to solve hunting in level flight.   But if you build it straight with motor down thrust and/or positive stab incidence, that dog won't hunt...

Ok thanks, so no slop with ball links it is..........