If you have the...
/cast Flash of Light (or Exorcism)
/stopcasting

In your click macro, it probably won't make any difference to you.
As you are automatically using it, every "x" seconds, if it has proc'd.

If you are waiting until you are notified of a proc and then proactively casting.
Then the notification end will be messed up some.
Note - Svpernova made a post today, regarding this exact issue and how to get around it.



*Edit*

http://www.dual-boxing.com/showthread.php?t=28667