Thanks for valuable inputs!
I think opto-isolator before TIP120 is very much essential in your design. Let me share my experience. For many days i was facing the strange issue of flash triggerring abruptly even though no signal was sent by Arduino. After lot of head banging, found that 1N4007 fallback diode after TIP120 was leaking small amount of reverse current which was causing my flash to trigger (since both are sharing same ground). After opto-isolating TIP120 my issue was solved. Now I am using separate power supplies for valves and Arduino with the separate ground.
Hope this helps for your next version of drop controller.