My £0.02: The FAA do send Acks but they can't be relied upon. For instance, I just filed a FPL in domestic format for a flight inside Texas, and didn't get one.
Getting the CID number is useful, but the best bet is to make sure that the crew get it in their paperwork, so that they can use it over the radio to the tower when requesting startup.
Putting the number of the relevant ARTCC in the paperwork is also a good idea... anything to reduce the amount of calls to Ops
