On Wed, 03 Jan 2007 15:35:41 +0100, Mxsmanic
wrote in :

I fly [MS FlightSimulator] 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?

Above 18,000' is Class A airspace. It's also called positive control
airspace; you won't get a "resume own navigation" instruction in Class
A airspace.

You can request lower from ATC, but until you are given a new
altitude, you must maintain that for which you are cleared.

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?

No.

Consider reading the appropriate Aeronautical Information Manual
sections http://www.faa.gov/ATpubs/AIM/, so that you don't pose such
obviously ignorant questions to three newsgroups.