The fine print of the code aside, I like things DEEP. For me, this means a minimum of being a coupl incehs deeper than the bottom of a slab, and deeper than the blade of a shovel (about 12").
Why? Because I don't want the pipe hit every time the gardner plants a flower or fixes a leaky irrigation pipe. I don't want the slab to act like "the worlds' dullest scissors" where the pipe exits. And I don't want the pipe cut when the plumber has to open up the slab to replace a drain.
