I've always been told that suspension parts should be tightened with wheels on and on the ground - a friend of mine changes his stock bushings to urethane bushings in his civic, after install, the car didn't sit level, it was from tightening the lower control arm and strut bolts while the car was in the air, we lifted the car again, losened all the bolts a little to take the load out of the bushings, dropped the car again, backed it out of the garage, pulled it back in, then tightened the bolts, everything was fine then. Also, urethane bushings squeak, it's the nature of the beast.