Do the transmission control valve shims play an important role and even risk preventing proper operation if incorrectly placed?
The shins are extremely important. They are used to set the initial pressure on each clutch. When a transmission has been overhauled (all clutches rebuilt) and the control valve has also been rebuilt with new springs, orifices, etc, it is pretty much guaranteed that the quantity of shims in each valve section will change. If the clutch pressures are not correct it is quite possible for one or more clutches to fail soon after the repair work.