This is one of those things where there are lots of differing opinions. Some who keep shrimp or other really sensitive items worry about any metal in the tank. Some are especially critical of brass or anything with lead content.
However, I come down on the other side and look at the metal as a relative thing once I consider how the water system is built. If we take a common system that pumps water from underground or a surface source, we know the water starts in a metal pump with brass parts. It flows through iron/steel pipes to the treatment plant where it runs through lots of metal while being brought up to drinking water standards. Somewhere along the line it runs in big metal pipes, sets in a big metal water tower for at least 24 hours to let the chlorine/chloramine have the dwell time needed and then on to the house through more metal and often into the household copper pipes and finally comes out of a faucet that is almost certain to be brass inside a pretty cover. It may set in metal for three weeks or more!
So do I worry about using a brass valve less than an inch or so long? Not too much!

 

Very good point, thank you.