In order for the 'Assigned To' field on cases to default to the user creating the case the role must be setup accordingly:
Go to Setup > Users/Roles > Manage Roles > select requested Role and click Customize > new page pops up with selected role
1. Employee Restrictions' must be set to one of the following:
b. 'Own and Subordinates Only'
c. 'Own, Subordinates and Unassigned'.
2. Main/Top section of Role > Support Role = TRUE
Note: The user creating the case must also be marked off as a Support Rep (Employee Record > Human Resources tab > Support Rep = TRUE).