For the private pilot course I did not teach instructor instrument flying until just before the solo cross county. This was to maintain the importance of looking out the windshield and using the natural horizon as the pilots solo reference for determining the attitude of the aircraft.
Since there was a possibility of getting stuck in low vis or worse in cloud during the solo cross country I wanted to make sure the student had the skills to escape.
Any instructor who does a pre solo lesson in weather that does not allow full control of the aircraft by visual means alone is IMO doing the student a grave dis-service.