If needed - torque to the recommended value, hold the wheel between your legs and try to turn the handlebar - if it doesn't move you probably won't benefit from friction paste anyway. If you find that in use the bar is moving off-centre then add a drop of paste, but bear in mind that slipping from a hard knock, e.g. a crash, is preferable to bending or breaking parts, as long as the slipping doesn't actually cause a crash.