[Bob Faulkner]

honestly, the best way to figure it out is to set everything flat, fade to a single pickup and play some notes and flip the switch for that pickup.. you'll hear the difference.. take that effect and just keep it in mind while blending the pickups and setting your eq...

in a nutshell, series gives you a more full and boomy tone from that pickup position while parallel gives you a more even response..

I tend to run the neck pickup in parallel to get the even tone from that position and the bridge in series to help balance the natural high end response from that position. And I run with the fader favoring the bridge pickup just a bit.

I'm sure someone else will chime in with a better description.