According to the FCTM
"PACKS
If the take-off has to be achieved without air bleed fed from the engines for
performance reasons, but air conditioning desired, the APU bleed may be used
with packs ON, thus maintaining engine performance level and passenger
comfort. In case of APU auto shut down during take-off, the engine thrust is
frozen till the thrust is manually reduced. The packs revert to engine bleed which
causes an increase of EGT to keep N1/EPR.
If the take-off is performed with one pack unserviceable, the procedure states to
set the failed pack to OFF. The take-off may be performed with the other pack
ON (if performances permit) with TOGA or FLEX thrust, the pack being supplied
by the onside bleed. In this asymmetric bleed configuration, the N1 take-off value
is limited to the value corresponding to the bleed ON configuration and take-off
performances must be computed accordingly."
So the answer is they are usually on unless required takeoff perfomance cannot suffer the slight loss of power of having the bleed on during takeoff. Hope this helps
