Sorry for that...it's clearer in my head.
The main problem with not having needle valves is that you won't be able to get all the bays to work the same. Sputtering may not necessarily be a problem caused by a lack of valves if all the bays sputter. It could be weak chemicals, too much air or not enough liquid.
I've found that with 3/8" tubing on the presoak, there's no need for needle valves on the liquid side. I have no trouble getting a wet, slightly foamy product to the bays with just the slightest bit of air, and for that the needle valves are necessary.
You can have two separate air solenoids for presoak and tire cleaner and still use the same tubing to the bay by teeing them together in the room. I use a cross at the boom to bring the presoak, tire cleaner and common air to the same check valve, so a separate air line would be a waste. Since the air needle valves are open so little on both, I see no need for bigger air line tubing.
Yes, 3/8" tubing only on presoak and spot-free (Actually two bays have 3/8" tubing for the triple foam since I wasn't getting enough liquid to them with the 1/4" and 100' runs). Our spot-free is medium pressure, 450 PSI, and 3/8" seems to be plenty.