You might consider renting a 24-70 f/2.8 for the event. If that isn't possible I would probably shoot the 50mm most. Try the 18-105 in low light, you may have trouble with the autofocus depending on how low the light turns out to be. I agree with the suggestion to get the flash off the camera if possible. Otherwise, bounce whenever you can. Bounce off the ceiling, off a wall to the left or right, even bounce off a wall behind you. The Rogue Flashbender is an affordable and handy accessory for a speedlight and IMO is better than a plastic cap style diffuser. If you shoot in aperture priority, watch the shutter speed - don't let the camera slow your shutter too far. There's lot's of movement at a reception. Most importantly, have fun with it! That will come through in your shots. Good luck!