Depends....
You want about 4 - 6* of caster in the front end. That means that the upper kinpin should be "laid back" more than the lower bearing.
Fortunately, most of the steering arms (at least the ORD ones I've used) are NICE and FLAT, so you can lay an angle finder right on top of them and measure what you've got now. Be sure the truck is on level ground, and the wheels are pointed straight ahead.
If the caster measurement is less than 4* (too negative), then the fat part of the shim needs to go in the front to help the pinion to come down and "lay back" the steering axis. If the caster measurement is too high (as mine was) you need to install the fat part of the shim in the back to raise the pinion and help to pull out some of the excess caster.
.