Ideally you shouldn't, but if the pump's maintenance isn't kept up, the spot free can back up through the valves and seals, it can damage the low pressure seals, and it can back up into the holding tank. Most washes have check valves in place to prevent backflow, but there are some that don't. I try to avoid putting check valves everywhere to reduce the chance of them failing. I cringe when I go into an equipment room and see check valves in five or six places on the high pressure pumps, on the bottom of every single solenoid stack, 3 or 4 (or the record I've seen, eight) at each boom.