Incorrect Prior to the scandal the governor’s approval rating was high.
Correct Prior to the scandal, the governor’s approval rating was high.
Incorrect At the end of the day money is just money.
Correct At the end of the day, money is just money.
A writer may use discretion to determine whether a comma is needed. Here is an example which creates a subtle difference.
Correct Of course I am happy.
Correct Of course, I am happy.