ENR 1.1 -18 10. ENROUTE
10.3.5 In non-radar CTA, pilots must report maintaining an assigned
level. An IFR flight operating VFR-on-top or requesting IFR Pickup
must advise level maintaining.
ENR 1.1 - 78 60. CLIMB AND CRUISE PROCEDURES
60.4 Pilots must report maintaining an assigned level.
60.5 After any en route frequency change, a pilot in command of an IFR
flight must advise present level. If the aircraft is not at its planned
cruising level, the pilot must also provide advice of the level to
which the aircraft is being climbed.
To summarise: Report reaching and leaving all levels, except that in radar on descent there is no requirement to report reaching a level, although you may choose to do so to encourage an onwards descent clearance.
Personally I prefer "approaching FLXXX"; but not when you're still 4000 foot above the level.
FTDK, As an ATC I'd be expecting you to tell me you're leaving A090, but wouldn't be shocked to see you doing so without a call, if I notice at all
. Usually when I'm giving a when ready descent it's based on you not having anyone close in front or below; i.e. if you comply with the descent at your discretion you won't take anyone out; a call telling me to look at you again gives me time to look at you re further descent/position re the steps etc.; it gets my attention again; but so does "approaching A060"; which I'd prefer than "Maintaining A060" (usually with gruff undertones implying it's all my fault).
Remember you are but one blip on the scope and my attention may be diverted on a different part of the screen (cause you aren't going to take anyone out); or having an in depth conversation with the tower about the plans with the departures (burst) or talking to arrivals about getting the next sequence better sorted than the last etc. Hopefully my scan will pick up that you have left a level; but then again it may not if it's only 1000 foot change etc.
Sometimes it's hard just because of the actual mode C displayed; A088 and A092 mean the same thing as A090; i.e. within tolerance. So just looking for a A089 isn't "proof" of commenced descent; usually A087 is good enough but it's amazing how many times you see A088, A087, A088 in consecutive paints.