You can shim it but it still does not take away 100% of the slop.
My suggestion is to get the steering as straight as possible. Make sure the links are perfect and identical in length. My car had terrible slop in the steering but when I drove it the slope was not noticable at all and the car tracked perfectly. Just make sure everything is assembled properly and that the toe is equal left to right...if it is, the steering will center itself when you are running.
You can keep worrying about the slop but you will never get rid of it.
I used misc Tamiya shims to take out most of the slop on the steering rack. Though keep in mind you won't get 100% of the slop removed and that's normal. Also don't go overboard and make it too tight which will bind the servo.