Not having gotten the stage where I need FoV yet I'm probably not qualified to enter this discussion, but I have started to think about it as it is the next thing I'm implementing after I've finished the various dungeon/terrain generators.
I am planning in daylight for the character to see the entire map cell they currently occupy. (barring tiles that are obscured by something else like a tree or a monster or a tree monster on fire). But the FoV around the character comes into play during darkness/twilight/dawn and gradually shrinks in around you at which point it will become radial.
I like to think of diagonal movement as 1.414 times as expensive as orthogonal (based on triangles and the square root of 2 blah blah blah) and this lends itself to a more radial field of view than a square one.
Ultimately as the OPs movement is the same orthogonally and diagonally it comes to a question of functionality versus aesthetics.
