I am thinking the paradigm has now changed of late...see bolded sentence...

http://www.ncbi.nlm.nih.gov/pubmed/21057122

Radiographics. 2010 Nov;30(7):1823-42.
Scoliosis imaging: what radiologists should know.

Kim H, Kim HS, Moon ES, Yoon CS, Chung TS, Song HT, Suh JS, Lee YH, Kim S.

Department of Radiology and Research Institute of Radiological Science, Gangnam Severance Hospital, Yonsei University, 146-92 Dogok-Dong, Gangnam-Gu, Seoul 135-720, Republic of Korea.
Abstract

Scoliosis is defined as a lateral spinal curvature with a Cobb angle of 10 or more. This abnormal curvature may be the result of an underlying congenital or developmental osseous or neurologic abnormality, but in most cases the cause is unknown. Imaging modalities such as radiography, computed tomography (CT), and magnetic resonance (MR) imaging play pivotal roles in the diagnosis, monitoring, and management of scoliosis, with radiography having the primary role and with MR imaging or CT indicated when the presence of an underlying osseous or neurologic cause is suspected. In interpreting the imaging features of scoliosis, it is essential to identify the significance of vertebrae in or near the curved segment (apex, end vertebra, neutral vertebra, stable vertebra), the curve type (primary or secondary, structural or nonstructural), the degree of angulation (measured with the Cobb method), the degree of vertebral rotation (measured with the Nash-Moe method), and the longitudinal extent of spinal involvement (according to the Lenke system). The treatment of idiopathic scoliosis is governed by the severity of the initial curvature and the probability of progression. When planning treatment or follow-up imaging, the biomechanics of curve progression must be considered: In idiopathic scoliosis, progression is most likely during periods of rapid growth, and the optimal follow-up interval in skeletally immature patients may be as short as 4 months. After skeletal maturity is attained, only curves of more than 30 must be monitored for progression.
I'd like to know what evidence they based the claim about 50* on and what evidence they are basing the 30* on.