+1 on the above unless the valve will be siting in the tank it's effects on the water are entirely negligible.
I was under the impression that the water going to the filter and heater would be routed through it? If so, then this would probably be even worse then it just sitting in the tank, since all of the water would be continuously passing through it.
I know there tends to be some fearmongering with metals, and that a lot of plumping, etc. is made of copper/brass/etc. But I'm not certain that getting water out of a tap is the same as dropping a piece of metal in the aquarium. Some of the water conditioners can help bind metals, and especially in planted tanks, (or tanks with driftwood or peat filtration) there are compounds that can bind the metals and effectively detoxify them. Apparently most aquariums seem to do fine with whatever comes in the water change, but I'd be concerned that something sitting in the tank (or having water flow through it) would eventually exhaust the aquarium's ability to bind/detoxify those compounds. I'd just rather stick to non-metallic elements if they are available.
And, it probably depends on what you are keeping. I think invertebrates might be more sensitive to copper and zinc.