The answer to this is no, categorically NO. You should never fix your Insignia shower cabin to the wall or floor in any circumstance. An Insignia Shower is designed to be moved in and out of situe as and when required. Fixing your cabin to the wall or floor could cause problems down the line.
If you level your tray prior to installing, the enclosure will be sturdy enough and will not rock back and forth. Fixing it to the wall is something only done when an install error occurs. If your fitter is adamant it needs to be fixed to the wall or floor, simply find a new fitter.