In this thread? I think you'll find that was me!
Downstream. It doesn't have to close on such a high pressure then. Actually it's built into the "primary" regulator, as is common for aquarium use. There are piccies of it in this old thread of mine:
http://www.thehomebrewforum.co.uk/showthread.php?t=64248
Further downstream there are more solenoid valves and the secondary regulators all hidden away in control boxes. If you need more evidence that you might be dealing with a paranoid lunatic here's a piccie of the "field" CO2 control box I'm building (with the "house" boxes cluttering up the rest of my table).
Guess I'm having breakfast elsewhere for a bit!