hello,
the decision to extend flaps will depend on:-
1. type of push-back equpment in use. if its of the type attached to the main landing gear then its prudent to wait till one gets the all clear.
2. type of a/c...i.e. ground clearance below the wing (B737 as against B747)
3.depends on how the flap/slats are powered. some can be moved only after engine start.hyd-v/s-elec.
So the sops vary according to the combinations.
the factor governing is safe operation