If your guitar has single-coil pickups, you probably cannot address individual pole pieces; they just aren't adjustable 99% of the single-coils out there. However, you can get an overall change in your sound by boosting or lowering your pickups either closer or farther from the strings. As you might guess, closer means they "pick up" (hence the name) more of the sound. That means they're going to be louder or softer.
This doesn't change your tone dramatically so much as it alters the basic volume level coming from your guitar. Why that matters is because your input level determines how much the signal will overdrive the preamp. Similarly, if you're using any digital processing and/or recording, your signal will clip (same principle, just sounds different). If you're going for a saturated sound like EVH or Metallica, you're probably trying to push as much signal down your stack as possible. However, if you're just looking for a bit of crunch when you really lay into the guitar (and not so much other times), then you might find that the pickups are more than hot enough. If so, back them away from the strings a bit.
The height of the individual pole pieces helps determine the volume of the individual strings. For single-note players moving from one string to the next in a solo or a riff, these changes can be quite noticeable. If the pole piece is especially close to the string, the increase in volume can make an ordinary note sound harsh or abrupt. Conversely, a low pole piece will make it seem as if the volume has dropped on the notes played on that string, thus sounding like the player is unsure or isn't fretting properly.
Another factor is the material. In virtually every case, the pole pieces are made with a ferrous metal that will conduct the magnetic field still farther upward toward the strings. However, different materials have different properties in this regard and therefore have the potential to yield subtle differences in tone. Feel free to experiment.
Messing with everything might give you an effect that is uneven. As I said above, sometimes you want more from some strings than others. This might be less due to pre-existing individual volume differences than to your playing style. For example, if you're doing a Chet Atkins-style bass line hybridized to rhythm and lead work on the high strings, maybe you want the latter to stand out. If so, you can back off on the pole pieces (if possible or you can raise one side and lower the other (i.e., create a differential across the strings).