Just to clarify, I would certainly not use or recommend concrete or tile grout to fill a block. universally everybody just seems to call it grout.
The scenario I described above is with hard block and I really wasn't as specific as I should have been about the procedure we used so here you go. I think the if the hard block is allowed to cure before the deck plates are installed that the cylinder wall will distort when plates are torqued and pull away from the filler. This defeats the purpose IMO.
My 400 block had plates installed before the filler cured, even though the machinist didn't want to do it that way.
We leveled the block, torqued the main caps, poured the filler in and used a vibratory device to shake the air pockets out, and then torqued plates, allowed it to set 48 hrs and then repated the same process on the other bank.