At the CLUG meeting of the 13th May 2009, during the presentation of Mike Richer of Park Air Systems, he highlighted a problem he was having when linking VIs, which have a cluster on the input, into a TestStand Step.
If the cluster was changed this affected the input parameter associated with the VI's cluster back in TestStand. When the VI prototype was reloaded the data contained in the Step's input parameter was set back to default values. This resulting in manually setting back the actual values used in the step.
Having looked into the problem I think I understand what is happening.
In TestStand a container is assigned to the VI's input cluster automatically, and is assigned the default values contained in the cluster. When elements of the cluster are removed then TestStand deletes the original container and assigns a new container thus it will have default values.
The attached MS Word document outlines the problem and offers two ways to avoid the problem or fix a pending problem.

Unfortunately there doesn't appear to be a tidy solution other than to use a variable as recommended by the TestStand Manual.
I hope this helps.

Regards
Ray Farmer