Don’t see the point & I’ve never done it; in fact under certain circumstances, movement in the tray could dislodge the bottom row of tiles. The tiles should overlap the top of the tray slightly so water running off the tiles is directed onto the top of the tray & not in the joint.what i mean about the shower tray is someone told me that before you put the tiles down u seal any small gap between the shoer tray and wall with waterproof tape.
Don’t grout between the tray & the tiles it will only crack & fall out; the tiller should have left around a 3mm gap between the top of the tray & the underside of the tiles. As I said previously, seal this gap in 2 stages; using a thin nozzle, inject silicone well into the gap between the tray & the tiles to provide a seal between the 2, scrape off any excess silicone so it’s recessed below the tiles & leave it to set. Then using a thicker nozzle on the gun (cut the tip off), apply another uniform bead of silicone over the original & finish off so it forms a nice radius bead between the tray & the tiles; this can be the tricky bit for the inexperienced.As for silicone, i always thought u grouted the very bottom vertical space between the tiles and the tray and then siliconed over that ? i.e. u see a bead of silicone around the botom of the tray when you look down.