Almost any lens will be less-than-optimal at its maximum aperture. That being said, there's a reason why the faster glass costs more -- a lot of work goes into getting that extra bit of glass at the edges to contribute as much as possible to image brightness while reducing the aberrations that contribute to overall image softness. That means, for instance, the use of aspherical elements and (often, but apparently not in the case of the Nikkor 50mm/1.4, which is not really pushing the speed limit) apochromatic correction (a technique to reduce colour fringing to ridiculously low levels, more often seen in telephoto lenses). The f/1.4 lenses are usually better at f/1.4 than, say, an f/1.8 (commodity glass) would be at f/1.8, and are almost always better at f/1.8 than the f/1.8 lens would be (there are older third-party lenses that are fast but otherwise abysmal performers all-around, and, quite frankly, the Canon f/0.95 was too mushy to use in anything but near-absolute darkness, but the Nikkors tend to be rather better than average).

You will almost always find that by the time a lens is stopped down two stops or so, you enter the range of maximum sharpness and definition, and the lens will stay in that zone until diffraction becomes an issue (starting between f/11 and f/16). That doesn't mean you have to stop down to f/2.8 to make the f/1.4 lens work well, just that it won't reach maximum sharpness and contrast until you get into that area. You won't notice any problems until you compare the result wide-open to something stopped down a bit unless you are trying to shoot something with very high contrast and detail.