When should I use the trim pots on the summing cards?

Trim pots are used to balance the output of multiple load cells on a single scale. Trim pots should only be used on floor scales when the product is not self leveling and the load sensors are not matched and balanced, or where the load can be placed in multiple locations.

Placing a barrel on a floor scale will be sensed differently by your load cells, depending on where you placed the barrel. With trim pots the load cell output can be tuned to provide the same output at any position. The default trim pot adjustment, or starting position, is fully CCW. This balancing measurement is different than adjusting for millivolt signal output. The millivolts can be all the same, but the load cell sensitivity will make a small weighing difference. Sensitivity differences is what you are balancing with trim pots.
You should never use trim pots to compensate for improper mechanical loading on load cells!
If this is a structure problem, shimming or load reorganization may be required. Trim pots should not be used on scales that have self leveling/centering loads. This could be a platform scale where the product being weighed is a tote that will only fit in one position on the scale. Try to avoid using trim pots as much as possible as they can be a source of noise. Adjusting the trim pots requires you to re calibrate the scale system. See the attached procedure to properly adjust your trim pots. 

C2® second generation calibration will not work on a scale using trim pots, with one exception. That exception is the older HI FS series platform scales that is adjusted at the factory for corner weighing and then tested for C2.