That's the eternal question for me, especially when painting white.
Generally you want to thin your paint by adding a few drops of water to it on the palette. If when you stir the paint you are leaving a furrow with the brush then it is too thick. The liquid should be the consistancy of milk, although I prefer a little thicker.
The problem is, the more you thin the paint, the more coats it takes to get an opaque coverage. I also find that if there is any dust that gets into the paint it will be more noticeable if your paint is very thin.
I guess you just need to strike a balance that works for you: paint goes on nice and smooth but retains enough opacity so that you aren't doing dozens of coats of the same colour.