Inclination of the test tubes increases the oxygen transfer capacity and is advised for aerobic systems with high respiration activity. You can find more information in the following publication:
Maximizing cell densities in miniprep-scale cultures with H15 medium and improved oxygen transfer
Danielson, P.D., Buechs, J., Stoeckmann, Ch., Fogleman, J.C.
Biochemical Engineering Journal (2004), Vol. 17, No. 3, pp 175-180