I have been doing a lot of rendering lately to present lighting layouts to a client. I have had to make my own light fixtures and use ies files from varying manufacturers. I have found that if you place the light source behind the glass in a fixture, the light will not show in a rendering. All I get is a point of light that appears to be on the surface of the glass and nothing is emitted from the fixture. When I pull the light source outside the glass, the light spills out and is reflected off surfaces around the fixture accurately. I then make the glass material self illuminating so it gives the illusion that the light is coming from inside the glass.
This seems to give me the best results in term of looking realistic for the client.