APU use regarding performance
The main reason to use an APU for take off is to improve take off performance. APU Bleed can be selected on during the line up leaving more bleed air for the engines. You would only tend to do this when the perfromance was critical.
(The other option is the checklist is to go Packs off on line upwhich achieves the same performance improvement.)
Most operators have APU Off for take off.