Lead Glass Windows - Tight or Loose?

I've had a couple leading classes from a stained glass teacher who said that the glass should be 'loose' in the piece. Loose meaning if you put your fingers on it flat before it is puttied, you should be able to slide the glass back and forth a little inside of its lead came (not loose enough that it falls out). The idea being that you should allow for expansion. But I have had a couple failures after my projects are done and I'm wondering if it is because the glass was 'loose'.
What is your experience when leading? Do you leave some room for expansion?
What happens if you have a 'tight' fit, ie the glass doesn't slide at all?

Milly's reply:
Good question Lisa. You don't say what your failures are, but I imagine they must be related to instability or sagging of the lead glass windows for you to link it to the tightness or looseness of the leading?
Your stained glass tutor is right in my experience - you don't want the glass to be totally rigid in the lead, as there will be no 'give' in the stained glass panel and it will be more likely to hold tension and crack. A bit of movement will also give you room for more cement, which will add to the stability of your panel.
However, you really don't want it too loose, as it will sag and be liable to crack too! As usual, there's a balance to be had. Good luck in finding it.