The 3 I've installed in my home, I spread a good glob of silicon on the flange before attaching the sink to the underside of the granite. This does two things: seals it, and helps to hold it in place. The granite fabricator I use mills T-slots in the granite, and you use T-bolts and clamps to hold the sink in place. If the sink is already glued in some manner to the granite, then it probably isn't an issue. But, if it is just clamped with nothing there, I'd take it off and then silicon it in place with the clamps.