A solenoid is essentially a check valve with a coil that pulls the poppet open to let flow through the opposite way. When it's closed it uses the pressure behind the poppet to keep it closed. D/H systems used the solenoids in reverse so the cold water rinse can't back up through the solenoid and into the chemical tanks. In the case of soap and wax supplied to a pump, the spring pressure on the poppet is enough to keep the soap and wax from flowing when it's not called for. It only requires a few PSI for pressure to force the poppet open, so you can't run them in reverse to control pressure. There's D/H equipment out there over 50 years old with the solenoids plumbed in reverse, so I don't think it shortens their life. I started doing the same thing about 20 years ago when I was seeing a high failure rate of the low pressure check valves on the soap and wax lines. Free fix and removes something that could fail.