Not much, usually less than 1PSI but it will depend on the valve. I had a couple of the duotight ones in my kegerator setup when I had a keg of water in there and I didn't adjust any pressures.Yeah ok. How much do check valves normally reduce pressure? Maybe I should just get another spunding valve...
If you are worried it is easy enough to test, wait until you have a known reading then put the valve in there and read it again to see the pressure drop.