cs: reference please
In PRO-NOR-SRP-01-70 ILS/MLS/GLS/FLS Approach it is not.
My training and was simple, definitely not to delay arming the APP mode until inside the ICAO envelope, or after "almost there" verification was made using other means.
Cleared for ILS approach -> press the button UNLESS in akward position. No need to wait even for angle to be less than 90 deg (737 habit - but I still do) .
. follows the philosphy "what's cleared (ATC) is set (autopilot modes)"
. reduces workload later and removes errors of omission
. IF you manage to hit and lock on a side-lobe, then ND and FMA gives clear picture of what has happened and corrective action is one pull of a knob.
Or, in other words: "When cleared by ATC and when appropriate" (this is FCOM PRO-NOR-SRP-01-70 ILS/MLS/GLS/FLS Approach)