Around here in Florida, they plan what all the plumbing should look like when it's all done, then just do it in place, brace it so it doesn't move, and pour the slab. Horizontal positioning is done using strings from reference marks on the batten boards or footer forms, and verticals just go up well above the finished height -- they're capped and trimmed to finish height later. I'm amazed by the precision -- pipes come through the finished slab right where they need to be when walls go up later, WC stubs are 12" from finished wall, etc., all to within 1/8" or so.
Supply lines are insulated where they're under the slab, waste lines are insulated to provide tweaking gaps between the pipe and the slab. Some waste & vent lines are sometimes left very long for static pressure testing. Needless to say, all pressure testing (supply and DWV) is done before the slab is poured. I'll look around for a home under construction (a rarety here these days) and post a pic if I can find one.
