This question gets asked over and over. And although it seems like an important one. It is basic common sense that answers it.
Place the mouthpiece in the center of your lips. Use 50% top lip and 50% bottom lip.
Now if it feels better to you a little off to one side of center that's fine. If it feels better to use a little more top or bottom lip that is also fine. However you want moderation as far as mouthpiece placement goes.
This goes for the distribution of mouthpiece pressure as well. 50-50.
Grip used to help break the habit of too much mouthpiece pressure. Grabbing the bottom of the valve casing helps (more mental than physical) to transfer some horn weight to the lower lip.
Stevens would have a player put his horn into playing position and then move it away from the lips until they separated from the mouthpiece. Then he would have you lock the elbows in place. To play you had to push your face into the horn instead of the other way around. Well you will not push as hard this way (mental). In fact you can't push as hard this way. This is one way to break the problem with pressure.
Pressure causes lip separation, swelling ..... Besides it holds the notes in. A player can be more responsive if using a relaxed setup.