Do we always put the main reinforcing steel in the bottom؟?

not always.depending on the structure we design (depending on the tension needs).

i.e. on cantilivres we put the main reinforcing steel on the top

The function of main steel is to carry tension force as concrete tension strength is 10% of it's compressive strength.

So, it depends on the structure like corbels, the tension is in the upper cord so, the main steel must be in the top

Deflection is highest at the bottom flange thus making the bottom of a structural element dense in steel. 

Not always. Depends on the type of the member that needs to casted, for example for a normal conventional slab the main reinforcement will be on the bottom whilst for a raft slab it would be on the top depends on the requirement of tension and compression steel.

we put reinforcing steel in the butoom for the main reason to enhance tension resistance in requested location , so it depends of what we have as structure and formwork , if we have for example a lost formwork using cold formed sheet plates like (lewis or HI-BOND55) there wil be no reason for adding reinforcement 

Well it goes into the bottom as well as as on the whole structure of your building or tower, that is the base and frame work of your structure..

Always we started by scrap metal and what the steel you like to product

no, it is depend on bending moment diagram and structural design due to loads of elements.

IT IS DEPENDS UPON DESIGN AND REINFORCEMENT AND CALCULATION.

No the reinforcemet will follow moment analysis of the structure and main reinforcement should be allocated at max moment areas 

the process of  provinding the main steel in any structural member is depend on the design of structural member i.e in cantiliver beam we designer provided the main bar in top surface.