Don't, unless you have a track car.

If you don't have a roll cage, and you flip your car... your head goes squash like a melon, because your body can't move to the side. With a regular 3 pt seat belt, your body can move to the side.

If you get a roll cage... are you going to wear a helmet all the time? If your head flies forward in an accident, and hits the bar, even a padded bar... your head goes squash like a melon.