Should rain gutters be installed behind metal drip edge?
If your drip edge (and the shingle protrusion) are working properly, water will fall from that outermost point. Obviously this means that the gutter shouldn't need to slip behind the drip edge flange. In fact, in cases where the run is long and the slope requires enough drop, this isn't possible anyway without additional flashing. That's not usually necessary, especially with newer construction and a 6:12 pitch.
Start at the high end of each run with the gutter just below the drip edge flange. Slope down from there. Nothing should penetrate or result in modification of the drip edge.