Depends on tension/gauge of strings... I use the thick ones starting at 0.13... so I use 4 springs set straight, just leaving the middle position empty. With just 3 springs, the strings pull the bridge too far up, rendering it susceptible to either breaking the bridge, or breaking some strings from whammying too far down (I have done the latter). I would tend to think using FEWER springs would be better for lower tunings, since there is less tension in the strings... that part is just a guess. So in my case, dropping to C might put me back at 3 springs for my 0.13 strings. ya dig?
...and I have three springs on right now but they still end up either buzzing out or making the strings too high if I take one off, I was just wondering if the arrangement of the springs in the back had anything to do with it, like right now I have one at an angle and two strait in the middle.