In a 4WD a slipper is there mainly to protect the transmission. For a car with ball diffs it is there to stop the diff from slipping if you gas it too hard off the line or landing a jump.
With gear diffs there is not the same need so I use it to stop it popping a wheelie
The advice in the Associated manual was to set it so it slips for a couple of feet off the line on full gas before locking up. As Darren said, this will vary from track to track.