All the action (see what I did there) happens at the bottom of the slot. Certainly once you get above the height of the string, it doesn't matter what width the rest of the slot is. But a V-shaped notch won't offer quite as much resistance to sideways string bends as a straight-sided notch. Obviously, the shallower the 'V', the less resistance there will be.

There is a strong argument for filing down the top of the nut so that the depth of the nut slots is minimal. I've seen just half the string height given as optimal, but to me, it's too easy to push the string out of the slot when bending near the nut. So I opt for the full string height, or just over. Leaving the nut high with deep nut slots means that the two ends of the nut are vulnerable to breaking off if you catch them (I've done this so I know). The two end slots only have half the width (or less) of nut material on the outside compared to in-between the other string slots, so are already comparatively weaker. If this end bit of nut is 3mm high (above the bottom of the slot) compared to say 1mm high, then you'll get 3x the leverage force on the material at the base of the slot if you catch it or knock it, making it far more likely for the end to snap off.

Filing the top of the nut down will also mean that cosmetically, the difference between straight and V slots will be minimal.