The way I see it is...
A. The spring should be long enough so that when the suspension goes into full droop the spring still has some load on it. (like you stated)
B. The spring should not be so long so as to bind when at full bump.
C. A softer spring with more preload is better than a stiff spring with less preload.
D. There is no "sweet spot" for springs. Most springs are constant rate so they can be run anywhere as long as the above parameters are met.
E. If you are using a variable rate spring, you will likely not want the "soft" part of the spring to be stacked or bound when at normal ride height. But this depends on the rate of the stiffer part of the spring. There are many variables to determine in the use of progressive rate springs.