5V is probably too low as the holding current it provides is barely sufficient. Basically you can calculate the holding current by measuring the coil resistance of your solenoid, and divide 5V by that resistance. For example, typical 24VAC valves have a coil resistance of somewhere around 25 to 35 ohm. Using 7.5VDC you get about 200 to 300mA, which is up to spec. If you use 5V, you will only get 140mA to 200mA holding current, which is a bit low. It probably still works — after all, OSDC will generate a high impulse voltage to energize the valve so that reduces the requirement for holding current, but still it depends on the specific valve you have.