The big problem with 100 watt incandescent bulbs in ceiling fixtures is the amount of heat produced and the possibility of them starting a fire by melting insulation on wires inside or above the socket. Most ceiling fixtures are rated for a maximum of two 60 watt bulbs -- especially if the bulbs mount sideways. If that is the case with your fixture, give yourself at least a little safety protection by placing two thicknesses of aluminum foil, shiny side down between bulb and fixture (not touching the bulb). That will help reflect the high heat from the bulb down and away from the wires. If however, the bulb mounts base upward and into a porcelain or high-temp plastic, you're reasonably safe. Just monitor the fixture for any signs of scorching. If a plastic fixture starts to yellow, it is because the heat is changing the composition of the plastic and making it more prone to fail. In such a case, discontinue use of the 100 watt bulbs for your own safety and that of other occupants of the building.