Confusion about when it's my navigation, and when it's ATC

Pixel Dent writes:

That doesn't mean you can't ask for a descent when your FMS tells you it
wants to start down, but there are a number of reasons you may not get
it. There may be conflicting traffic below you or the controlling
authority for the area you are in may have letters of agreement with the
surrounding areas that specify what altitude flights heading in various
directions or to various destinations must be at. In addition sometimes
the controller you're talking to doesn't control the altitude you want
to head down to. For a given ground position there may be three
different controllers handling different altitude blocks and your guy
may not have time to coordinate or even be allowed to descend flights
into the area below you.

OK. But this is a bit worrisome in that the FMC apparently already
calculates the optimal descent. If I can't start down in time, I may
not be able to make the other altitudes in the route without holding
or some other diversion to provide more time to descend. For some
reason, the FMC seems to be optimistic about how quickly it can force
the aircraft down. I have to deploy spoilers a lot, and that's even
when the descent starts on time.

Anyway, so I can just say "request descent" when I'm ready to start
down?

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