individual ethernet cable wire uses
They do have some differences in the pitch (the amount of twists per distance,) however this is to help prevent cross-talk between the pairs by making sure the same conductors of the different pairs don't repeatedly lie next to each other over the entire length of the wire. 

Green and Blue are the most twisted then orange and finally Brown is twisted the least.

You need to use each twisted pair with its polar opposite. Normally each pair carries a differential signal so that any interference is cancelled at the receiver, but I'm not sure exactly how you're going to be using it. 

For example, the blue and blue stripe should be +5v and -5v (or ground.) This will help to cancel noise and help to reduce the cable from producing noise that may interfere with something else. 


  I remember reading that because of the loops in the wire and the
  impedance effect some pairs were best for data and others for power.


It is best practice to use the blue and green pairs for data and the brown for power. You can also use the orange for power, especially if you are pulling a bit of amperage. You can couple the orange and brow solids together and the orange and brown stripes together.

Be careful to not use to long of a run and make sure the device powering the serial can provide enough power or else you may damage you device.