If we assume the pump will keep the fert out of the water line (sounds like untested agreement on that point), then I would definately want the valve for safety - not finding the tank siphoned off onto the floor.
But it does raise the question of "at what point do you want to separate the fert from the water - keeping it from mixing?" That can be either at the pump, or at a check valve (if used).
Since a "dose" is anything that has left the pump, I would assume thar you would want everything that has left the pump to be in the water. Even if that last bit left in the feed line to the T just seeps in after the pump stops, rather than coming in under pressure. It's still part of the dose.
But to do that you have to eliminate the check valve, raising the safety issue again.
This is a quandry.
Steve - 33g reef and a 180g planted in need of a re-scape.