I am a DVC Member and Bay Lake is my home resort.
During one vacation, prior to both Grand Floridian & Polynesian DVC expansions, we watched the fireworks from each resort.
Bay Lake/Contemporary is the 'worst' view because you are watching from the side, not towards the center of the fireworks. The closer on the Monorail Loop to Magic Kingdom, the better the view of fireworks.
So as previous post mentioned, I would not make a CA Grill reservation just to watch the fireworks.
Then again, if this was pretty much my only option (since returning to the Parks do not always work with young children), then this would be OK too.
Really, kinda' depends on your circumstances.
Just a note on the positive side - some rooms at Bay Lake that are considered Lake View may also have a reasonable viewing of the fireworks. On this same note, you can 'tune in' the simulcast on the TV in your room - just as they do from CA Grill and all the other dining locations where the fireworks are viewable (i.e., Narcoossee's). We did this last year when Observation Deck was at capacity. Try to request, "an even numbered room in the North Tower," for best result if you have a Lake View; any room with MK view is about as good as Observation Deck.