Are you talking about a solenoid or a check valve? I'm injecting dowstream of the throttle so I use a 1 bar check valve insert in the jet to stop it sucking water in when the throttle is closed. I think pumps generally like blowing rather than sucking so it makes sense to me for the check valve to go downstream of the pump, as mine is.
People seem to use solenoids to purge air from the pump, and I guess if you're doing this it would need to be downstream of the pump. But if you are just using the solenoid instead of a check valve, there might be advantages to putting it upstream of the pump because that would prevent the check valve from being exposed to full injection pressure.
Not a very good answer I know, but something to think about until the experts turn up.
