Robert,
You would use the higher pressure settings so you could get more heat out of your torch--for example, I often use the higher pressure settings when I work boro. If you were making boro marbles, pendants, or anything sizeable it takes much less time when you have more gas pressure/fuel available.
If you're working soft glass, you would definitely use the lower of the recommended pressures.
Anita
www.fireflybeads.com