When to change out the fly wheel?

I need to pull my trans and put a new clutch in. I would like this to be the last time I have to do it for a while. I have a 340 4 speed. How do you determine if the fly wheel needs to be replaced? How often when replacing the clutch would you need to replace the flywheel?
If you have the Flywheel machined. They should check it before and after. I believe there's a Spec. stamped right on it. I've always just installed the new Clutch/Pressure plate/bearing without machining on My own vehicles. The main risk is "chatter" or vibration.