I don't think that every event is equally important or that any event is "special". Rather, the avoidance of paradoxes is. You just have to ensure the alternate history you get with respect to an action (or an event) doesn't preclude that event from happening. So if you go back to stop a disease from escaping a lab in Texas, that doesn't stop the disease from escaping two months later from a lab in California. The key is the avoidance of paradoxes.