We were unable to efficiently use MCX (in Casper) to manage dock items/order so we found a few alternate methods. As far as I know, Casper is not able to perform dock arrangement in MCX.
Which OS(s) are you applying the dock to?
With Casper 8.4 you can create "deployment profiles" that set up just like an iOS profile and set your order that way.
Using a Casper "dock item" policy does not set any order to your dock. If you run the policy on all of your machines at the same time, you will get similar results, however if you run the policy on 10 machines then wait an hour and run it on ten more, you will most likely get a very different order.
Another possibility is to build the dock on a test machine and capture it using composer with the "user environment" option. This will capture your order as well.
Depending on when you want to apply dock changes in relation to user creation there are other possible issues that can arise.
Hope this helps a little!
Pat B.
Springfield Public Schools