The stationary stops on one side of the platform will automatically support two corners.  The other two corners will be supported with the sliding stops.  I think I'll have good support, but I won't know if racking will be an issue until I've got the tools loaded.  

One of the other design criteria is for the planer bed and the drum sander bed (the tool opposite) to be at the same height as my workbench top so that it can be used for outfeed support.  Due to the max height of the drum sander, I'll most likely have to outboard the casters in order to drop the cart low to the ground.  This will also allow me to maximize the height of the bracing at the lower back, so hopefully side racking won't be too much of an issue.