In most situations, I would agree here, and prefer to get the Conduits at least 2" below the bottom of the concrete slab.
Sawcutting dudes are always talking about the sparks flying out of underslab conduit runs! Some have been shocked to an almost lethal extent! (if I was doing this work fulltime, the first nibble would be the end of my sawcutting career!).
The most common scenario of sawcutters vs. underslab conduits is in the pourstrips for Tilt-Up Buildings.
Seen sawcutters blast through Subfeeders and the area lighting circuits (for wallpacks).
The Subfeeds pop really hard (sawcutting personnel usually take a break and check their pants!
The HID wallpack circuits do nothing, until the timeclock kicks in the lighting contactor! That's when you hear the familiar roar!
For most underslab runs - used to feed Floor Boxes containing Receptacles + Comm/Data - I prefer to drop as low as possible. After inspection, backfill and compress, until back to proper level. Then drill and install Rebar "Pins" in the cut areas.
When Walker Duct (or similar) comes in to play, there is nothing else to do except place it in the finished (repoured) concrete slab. however the conduits feeding the pullboxes can be swept down below the slab bottoms.
Now for the exceptions!
In areas with Flammable Vapors seeping upwards from the Earth, the need to keep an unpenetrated Vapor Barrier just below the concrete slab is encountered.
Vapor Barrier consists of heavy visqueen (sp?) plastic with Sand top cover and (I seem to remeber) fine peat gravel cushion layer underneath the plastic. it's been awhile, so don't quote this directly!
Working around this presents a new meaning of the word Tedious!
One certain area in Southern California has an extreme problem with Vapors seeping out of the ground and collecting in "Not So Preferred Places" (i.e. conduits and such! explosions waiting to happen!).
The Huntington Beach / Huntington Seacliff area is prone to this.
(Electure and Nick should know right away what I am getting at here!)