The driveshaft angle is somewhat limited by the maximum available in the u-joints being used. Exceeding the limits will cause the u-joints to bind.
Some angle is needed to enable the u-joints to function as designed. The overall life of the u-joint will be greatly decreased when the driveshaft angle is at or near the maximum available for the u-joints. Extreme angles also will limit the working RPM.
For most street vehicle applications the transmission/driveshaft/pinion gear angle should be in the 3-5 degree range when at rest and at ride height for best performance and life.