The important thing with fiberglass insulation is that you don't want to compress it. If you do the R value will be less where it is compressed.
With that in mind a good insulation installer will do a knife cut partially through the batt from the backside to put a horizontal cable in the center of the insulation without compressing it. The same can be done for vertical runs only the insulation can be split with your fingers to accommodate the cable.
I like to see the space behind a box (between the back of the box and the sheathing) filled with uncompressed insulation. You again would split the insulation with your fingers, slide a layer behind the box then cut the front layer away around the box. I think that the thin uncompressed layer behind the box has a higher R value than the whole thickness shoved in there.
Light fixtures in insulation should be type IC so they can (and should) be completely covered following the same rules I gave above if applicable.
-Hal
