Unsure of a situation that came up today:
IFR in IMC flight enroute to a nontowered airport, filed TAS was 132.
I was give instructions to fly to the published hold for the ILS with an EFC time of over 30 minutes for traffic sequencing. In order to conserve fuel I slowed from 120KIAS/~132KTAS down to 80KIAS/90KTAS approx. 6nm from the holding fix. I reported to the controller my change in TAS from what I filed.
Practically, I'm sure the controller didn't really care whether or not I did or didn't report to her my change in airspeed, but, am I legally required to report this change in TAS as it is more than 10KTAS or 5% filed TAS as I'm not really "en route" anymore but establishing an entry for holding and didn't see the purpose doing so at my max fuel burn?