Well, I would use the neck for leads when playing blues.
I would use the bridge for lead and rhythm when playing rock.
The middle 3 positions I don't know much about because I play 2-pickup guitars.
I guess that would be up to you to decide what they sound best for.
On my guitar I use the neck pickup (Charlie Christian) for lead and rhythm because it's so warm and versatile. My middle position is wired to be out of phase so I use that for when I REALLY want to cut through the other instruments like a hot knife. I use my bridge pickup (SD SH-4 JB) when I'm playing distorted rhythm or if I am doing pinch harmonic leads.
=]
Dm
"Talent instantly recognizes genius,
but mediocrity knows nothing more than itself."
-Sir Arthur Conan-Doyle