As far as I can tell, the Enterprise's navigational deflectors are spherical. Otherwise she couldn't accelerate in full reverse or do warp pivots or later on in the movies fly around V'ger.
Well, I guess I should have qualified which sources I meant. FJ has the navigational deflector on the main dish, and that faces only forward. Memory Alpha concurs: http://en.memory-alpha.org/wiki/Navigational_deflector
It's very difficult for me to reconcile the idea of a dish that aims in a particular direction with the idea that deflector beams can be directed in any direction with equal ease.
But what of the Reliant? Or the older ships like the Aurora Space Cruiser? They don't have a dish.
The first time the "dish" = navigational deflector on screen I think was in TNG with the E-D. I just view that as a limitation of the E-D and ships that used that design philosophy which the TOS Enterprise, movie Enterprise, Reliant, Stargazer, etc did not follow.