Q:

Why is the deflector disc for most starships below and behind the front of the ship?

One would assume that you would want it in front of the ship. As is the case with the "USS. Enterprise NX-01" Also why is there only a front facing deflector, when the ship can travel along several different axis.  
The dreadnought USS Federation "also" has a rear-facing deflector dish(!) 'In the diagram the notation on the "main sensor dish F/A" the notation (F/A)=(Fore, & Aft); on the Star Trek USS. Enterprise Original Blueprints it reads "main sensor and navigational deflector"  I have always wondered about that; is there an actual explanation for this?

For example; in the TOS episode "Balance of Terror" the Enterprise goes to "full reverse", or in the Wrath of Khan when the Enterprise goes straight up and then back down again. What protects the ship during these maneuvers?  
Because the shield are not always on??? right? or am I confusing deflectors, & shields?
Funny thing well for everyone but me. I have the technical manual, I do not remember seeing that in there.

A:

The ship's deflector dish needs a clear line of sight along which it can project a deflection beam, hence why it needs to be at the front of the ship. Its exact placement is largely irrelevant. The deflector in the Galaxy-Class is below the saucer whereas it's right at the front of the Defiant-Class.

Star Trek: The Next Generation Technical Manual
Since the navigational deflector only operates when the ship is at warp (and since the ship can only warp forward, not backward), by necessity it needs to be at the front, rather than the rear. 
When the ship is at sub-light speeds, the deflector tends to be turned off and the ship's shields take care of low-speed impacts.

For the record, the USS Federation (which appears briefly as a background image in Star Trek II) also has a deflector at the front. What you've mistaken for a rear-facing deflector is in fact a secondary sensor array. 

Star Trek Star Fleet Technical Manual

