The best time to do Inner Safety Meditation is when you feel reasonably calm.  If you are experiencing high anxiety, it is best to use your cognitive techniques (see Mastering Panic) and/or Belly Breathing.

By meditating when you feel reasonably calm, you are not asking your Inner Safety Meditation to compete with established patterns of anxiety.  By using your cognitive techniques when you experience anxiety, you are directly addressing and resolving the anxiety, a little at a time.

How do meditation and CBT fit together?