Here we don't put stone under the pipe unless it gets off grade a little bit.  We use ASHTO 57 and enough to just cover the pipe and to keep stone costs down we try to use a 1 foot bucket.  I remember hearing that years ago they would put staw on top of the pipe, but, apparently there were a number of times where they found mice nesting up in the straw and they would chew through the pipe.  Whether true or not, I've seen only one job in 25 years where the farmer put straw on the tile

ETA: I am referring to 4 or 6 inch tile..