Now that you know why you need to rake leaves, the question becomes one of when to do so.
A general consensus is that leaving leaves on the lawn for more than three or four days may be unwise. That said, other factors must be taken into account, too. How thick is the layer of leaves? Have the leaves been matted down by rain? The thicker the layer and/or the wetter the leaves, the sooner you should rake or otherwise remove them.
How many leaves you anticipate having to rake up should also be weighed as you determine when to rake leaves. Large yards with lots of deciduous trees obviously pose a bigger leaf-raking challenge than other yards. People who own such yards may have to start raking earlier, just to ensure that they don't fall behind in the leaf-pickup project.
Another factor that might influence your decision on when to pick up leaves is the leaf-removal equipment that you'll be using. If you use a leaf blower properly, you may be able to pick up the leaves faster than you would using a rake, allowing you to procrastinate a bit longer. But using leaf blowers isn't for everyone.
One time-saving method for leaf removal that may preclude having either to blow or rake leaves is to use a mulching mower, instead.