"God may choose to heal or not to heal. "
Support for this doctrine is not found in the Bible. They may have taught otherwise at seminary or this may have been your experience but this is not consistent with Jesus' ministry on earth.
If you disagree, then please find this doctrine in a relevant passage and show it to us in context, keeping consistency with the ministry of Jesus and the doctrinal statements which New Testament writers recorded.
Isaiah 53:5 is developed more fully in 1 Peter 2:24.
The healing described in Isaiah 53:5 is spiritual healing of the soul from sin.
Christ's healing ministry was an authentication of his identity and mission as Messiah - (Lk. 7:19-23; Jn 5:36; 6:14; 7:31; 9:30-32; 10:37-38; 11:42; 14:10-11; 15:24; 20:30-31; Acts 2:22).
So too, the healing ministry of the Apostles was in order to authenticate their message prior to the finalization of the New Testament Canon and the final destruction of the Old Covenant in AD 70 - (2 Cor. 12:12; Acts 2:43; 3:1-8; 4:33; 5:12; 9:36-42; 19:10-12; 28:8-9).
Even in the Apostolic era, not all Christians had the 'gift of healing' or 'gift of miracles'. For instance, when Dorcas died, Christians called in the Apostle Peter from another town and he, an Apostle, raised her from the dead - Acts 9:36-42. When the apostles died, the sign gifts ceased.
A handy article by John MacArthur can be found below:
http://www.biblebb.com/files/MAC/sg1858.htm