Ideally in watercolor you leave the white of the paper for anything white in a watercolor painting. However, from time to time you may need to add a little white accent here and there, a little white flower, a bit of white splatter to add texture. Sometimes, it's impossible to leave the white of the paper and you need a touch of white paint.
In most brands of watercolor paint, chinese white isn't really opaque even though it may say opaque on the label. It is more like semi-opaque. A better choice would be either white gouache which comes in a tube just like watercolor paint or Dr. Martin's Bleed Proof White which comes in a jar and is a liquid opaque watercolor paint. Gouache is pronounced "gwash," it is an opaque watercolor that has a flat finish when dry.
On my chart to the right, I painted a swash of chinese white, two different brands of gouache and bleed proof white both in the left column directly on the paper and in the right column over top of a dried swatch of brown madder paint. You can see how two coats of chinese white still shows up as semi-opaque.
For those little touches and sparkles of white, I would recommend using titanium white gouache. Leave it in the tube and put a tiny little dab on your palette when you're ready to use it. Mix in a little water to smooth the consistency. It should also be the last thing you add to a watercolor painting. The only thing you should add to a painting after the white gouache is your signature!