What kind of thermostat do you have on the car? If it is a Behr Thomson than replace it with one made by Wahler 75°. Brake off the check valve on the thermostat.
Fill up the radiator by taking off the top radiator hose and fill through there until you see the water come up the water pump. This eliminates the possibility of air pockets in the radiator, assuming you have an original Behr radiator.
Yes, you do need the thermostat. Without the engine runs too rich, you will loose a little of your fuel mileage, and it can damage the catalytic converter over extended driving without the thermostat.
Good luck;
Jean