OK, so it’s not as famous as e=mc2, but here is the rule I apply for when to test an application for app compat.
You should test your applications when:
cost failure × probability failure > cost testing
You should otherwise not bother to test your applications – just fix them reactively through your standard helpdesk processes. From a business perspective, it just doesn’t make sense to pay more to avoid a risk than it would to just march straight into it and have to pay that fine. I mean, if I offered you a “get out of traffic court free” card for $1000, and traffic tickets cost $100, how many would you buy from me?