I used one side of a Stainless Steel cotter pin to cover some poles/pins to experiment with. The flat inside of the cotter pin sits on top of the poles nicely. A little more metal to them too.
Basic electronics, the way pickups work is the vibration of the metal strings causes a change in the magnetic field, this produces an electronic signal in the coil which when amplified produces the sound.
That being said depending on the strength of the magnet, will determine how far the magnetic field extends out from the center of the magnet.
So as long as the string vibration is in the magnetic field it will produce sound. The advantage of having the string centered on the magnet is you will get a higher electrical change in the coil.
This is the reason that pickups are adjustable in guitars so you can tweak them to produce the sound you want. Closer to the magnet, louder sound, alternatively a bigger distant will give less sound. Not sure if it changes the tone too.
I've always heard that about the strings needing to be directly over the poles too, but they can be slightly to the side and still work good. Case in point: The Fender P Bass has the strings pass directly between the poles. So as long as the strings pass between the mounting screws on the ends of the pickup your probably okay.
Often debates came up about "drop out" when a string was in a bend and was no longer over the pole. I never found that to be a problem. I've seen many a guitar where the strings were only close and still work. I think people mistook the drop of sustain in a bend as the string not being over the pole resulting in drop out creating the misconception.
Tone is also changed when moving the pickups up/down. Up=brighter tone Down=darker/mellow tone.
Danni,confusion reigns when terminology is muddled,a single pole pup is essentially a pup with a single magnetic pole orientation,as opposed to a humbucker ,which may have 2 or more poles of opposing polarity,i#m not explaining this well,but,the individual pole pieces of a pup,are not indicative of single pole or multi pole construction,someone else will help me here i hope,but basically the individual posts under the strings,are not an indicator to being a single pole pup or otherwise in themselves,and yes,most definately,if you were to place a pup,beside or angled to the strings,it WILL effect how strong a signal is sent to the amplifier,generally your posts /or rail should cover all strings equally,unless you are deliberately meaning to emphasise either treble or bass output