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.