Yes.
1.Different pieces have different armor values, Chest -Highest to Belt Lowest.
You usually want to have large pieces heavy or medium if you go 5light/2 Something else. You want to have small pieces light if you want only 1 or 2 of them in your set.
2.Large armor pieces(Chest/legs/helm) get the full value of a glyph when enchanted while small get only a fraction of that value.
3.Dropped sets can have different traits on each piece and since some trait are completely useless the choice is important if you try to optimize.
That depends on your cup size, I imagine...because a helmet doesnt fit your chest as comfortably as it does your head.