Packs on Engines is standard unless T/O performance requires the Engine Bleeds to be selected off.
If the T Flex goes below 45°C we are recommended to switch Engine Bleeds Off to reduce Engine wear.
In these cases 'Packs on APU' or 'Packs Off' is crew discretion. If the flight is booked well and/or it's hot outside APU might be the better choice.
After De-Icing we normally use 'Packs Off' in order to avoid the run-up which was necessary to clear residiual fluid off the engines. If there is a longer delay between end of de-icing and take-off 'Packs on APU' is a lot more convieniet to the pax.
Regards, MAX reverse