January 20th, 2012,10:20 AM
It just depends on how much Roto and tracking you will have. If people are dancing with long curly hair in front of that thing, you probably want to green it out. Likewise, if you are in so tight on some shots that it's all mirror, unless you find some specs of dirt to track on to, you're in trouble for a 2D and/or a 3D track. If you're out wide and see the corners, you'll be fine.
Also, remember, a mirror is (more or less) a 100% reflective surface, whereas a window made of glass is a more complex surface with some reflection, but also refraction of light coming in from the outside (due to the imperfections in the glass). There's always some atmosphere that gets caught in there, occluding some of your reflection. Not to mention if it's dirty glass you get some light pick up and occlusion on that dirt.
You could always put some green screen in front of the mirror, then get some glass cut to that size and put that in front of the green screen, that'll get you some reflective elements as well as the shot at a key.
Oh, and watch for camera reflections
VFX Creative Director // Encore Hollywood