well, if all you plan on having for mids is the 2 speakers, then just the 2 front channels would be fine. if you plan on going 4 spekers, or putting a sub in, in the future, then you might want to put the 2 runs in at the same time, and leave the extra set taped off somewhere safe. when it comes to fading, and ballancing, it simply turns one or the other down, rater than adding volume. your audio is going to be 2channel anyways, so you will not loose anything that the rear would be playing. depending on your head unit, later, you may be able to run the rear channels as sub out, and the front as mid/high. for now, either is capable of full-range, identically. unless you are referring to multimedia systems that have surround sound processing for movies, the only effect you would be seeing with the "back-ground effects and noises" would be the fact that most factory full-range speakers have the larger speakers in the rear, like 6x9's run infinate baffle to the trunk, and the fronts generally don't have as low of a frequency response, so you can hear more bass-notes, or less mid, etc. either way, it is a difference in individual speaker properties, not the signal they are getting