Even on a 12 megapixel 5DC, using a tripod, cable release, and mirror lock are a must when using my 70-300L at 200-plus mm focal lengths with longer shutter speeds. I can still get reasonably sharp images without the mirror lock, but for tack sharp images locking up the mirror is a must with longer exposure times. I'd think that people that are so adept at pixel peeping at 100% looking for things like chroma noise would be able to spot this sort of thing.
Technically speaking, when the spatial res. of the sensor the same as or less than the spatial res. of the lens, the more likely you are to get what appears to be a "razor sharp" shot. That apparent sharpness comes at the cost of resolution, though. Sharpness and resolution are disjoint things...one refers to how fine the degree of details can be resolved, while the other refers to the clarity of those details (MTF refers to both of those things at a given contrast ratio, which is what makes MTF valuable.) You can get "sharp" images at any resolution (if you want to get extremely technical, a 1 pixel sensor could take a sharp shot of anything, it just wouldn't be a useful image.)
The real question is whether you can get sharp shots at the resolution of your sensor. Inside of the "band of maximum resolution", which is maybe a 1 2/3rds stop area around the f-stop a lenses resolution peaks at (usually f/4, 173lp/mm, lets call that the ideal aperture), the sensor should always be sharply recording whatever the lens is projecting. Assuming proper exposure and generally following the 1/focalLength rule for camera shake (including the use of IS/VR lenses):
- Softness below the ideal aperture will usually be due to the lens...optical aberrations like chromatic aberration, longitudinal fringing, spherical aberration, etc.
- Softness above the ideal aperture will usually be due to diffraction.
At apertures above ideal, you need to get about 2-3 stops beyond the initial DLA of the sensor before that really becomes a problem. If a sensor's DLA is f/8, it wouldn't be until about f/16 or possibly even f/22 that diffraction visibly affects the image. (Even at f/22, diffraction is often likely to have LESS impact on IQ than optical aberrations at say f/2.8 or wider when the ideal aperture is f/4.)
Consistent softness at all apertures can occur when the sensor significantly outresolves the lens. When the sensor outresolves the lens, a single line pair resolved by the lens will usually cover more than just two rows of pixels (or four, in the case of bayer sensors). You can usually realize improved resolution up to the point where the sensor outresolves the lens by about two fold (and even farther, however the cost to produce such a sensor will generally outweigh the cost of a better lens, and a better lens will help you realize greater gain than a better sensor at that point.) You may not, however, continue to experience improved sharpness as you increase sensor resolution. If you keep improving sensor resolution without improving lens resolution, the sensor is going to consistently start resolving enough detail to show the attenuation (...falloff) between line pairs, so instead of high acutance (strong contrast along the edge between a light line and a dark line) you'll get lower acutance (kind of like using a gausian blur along the edge between a light line and a dark line, creating a smooth gray transition rather than a hard edge.) Technically speaking, you are still resolving more detail, its just that beyond a certain point, the detail you are resolving appears less perfect (less sharp) than at a lower resolution.
In the band of maximum resolution, there is still a lot of room to gain more resolution with higher resolution sensors. However at other apertures, and in the corners of lenses, sensors are already outresolving lenses to some degree. At f/22, lens spatial resolution is limited to 68lp/mm MTF50 (good contrast...i.e. decently sharp). The Canon 7D with 4.3 micron pixels (116lp/mm), and the Nikon D800 with 4.8 micron pixels (102lp/mm), are getting closer to outresolving f/22 resolution two fold (136lp/mm), and are likely already outresolving f/2.8 two fold. Possibly even more so, as 900lp/ph with FF in lp/mm would be 900lp/24mm, or 37.5lp/mm. The D800 is capable of 2.8 times that, so its nearly 3x outresolving f/2.8 from the LensRentals test. The reason images might look slightly soft (i.e. not tack sharp) with a high resolution sensor like the D800's or the 7D's is because the sensors are outresolving the lenses at most apertures.
If you expect tack sharp photos strait out of the camera at any aperture (and don't print large), you are actually better off with sensors that have lower resolution. Ideally, a sensor capable of 68lp/mm would be tack sharp all the way from ideal aperture to f/22, and would only doubly outresolve f/2.8 (so it might not be tack sharp, but it should still be sharp enough.) The 5D Classic sensor has a spatial resolution of about 61lp/mm. The 5D II has a spatial resolution of 78lp/mm, the 5D III has a spatial resolution of 80 lp/mm, and the D800 has a spatial resolution of 102lp/mm. I would expect all three 5D cameras to produce what appear to be sharp shots more often than the D800 in hand-held situations. The D800 is more demanding of lenses, and it will be outresolving more apertures to a greater degree than any version of the 5D. I'm not sure I would go so far as to say you need "perfect" technique, however having better skill at hand-held photography, or the use of a tripod, will generally benefit the D800 more so than the 5D III.