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

I fly from KLAX to KLAS, using the FMC to handle most of the flight.
With the routing I put in, the FMC decides on some default altitudes
and includes required altitudes for the arrival and departure
procedures I select. Part of what it does is to create a descent
schedule from the nominal cruise altitude to the arrival procedure.

So I leave KLAX and my last explicit instruction from ATC is "climb
and maintain FL290," which is my programmed and filed cruise altitude.
Now, my question is this: If the FMC has a programmed descent in its
route, do I let the FMC start the descent where it sees fit, or do I
force the aircraft to maintain FL290 until ATC explicitly clears me
for my own navigation or for a lower altitude? And if ATC's last
instruction had simply been "resume own navigation" or "proceed as
filed," would that mean that I'd be free to begin the descent whenever
the FMC (or I) decides it's best?

In situations where I can begin the descent at my discretion (assuming
that own navigation implies this), should I tell ATC that I'm leaving
my cruise altitude? If the FMC has a continuously changing estimate
of lower altitudes in the descent profile, what should I give as my
target altitude? The next fix that has a specific altitude? (Such as
a fix in the arrival procedure)

Climbing I think I understand. If I'm told to resume own navigation,
or cleared as filed in the first place, I climb per my flight plan/FMC
profile. If ATC says maintain X, I stay at X until ATC tells me to
resume own navigation or instructs me to change altitudes. But the
descent part still has me a bit confused.

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