I think this more answers the question "when can you raise an A" than "when should you raise an A".
Series A and later are really helpful in growing quickly. You should raise a series A when it will give you a lot of growth per dilution -- that is, when you have a clear plan for how the money will help you grow quickly, and some evidence that your plan will actually work.
That was overly simplifying and I don't believe for a second Aaron literally believes you should raise any money you can simply because it's available. The downsides of raising money you don't need are something tons of founders, YC partners, and VCs have all discussed before.
Series A and later are really helpful in growing quickly. You should raise a series A when it will give you a lot of growth per dilution -- that is, when you have a clear plan for how the money will help you grow quickly, and some evidence that your plan will actually work.