Does anyone know why the 50x figure is often recommended? Personally I use the distance I normally use the lens at most. For example 50x for a 100 macro is 5 meters and on the odd occasion I use it at that distance f/2.8 isn't that focus critical, for true macro I usually manually focus so I set mine at about 1 meter which tends to be about the range I use most for quick handheld snaps where AF is important.
Its fine to use it at your commonly used distances. 25-50X is recommended because that provides a wide range of correction. If you move much closer than 25X, many lenses will start to front focus, so if you always shoot close, thats fine, but longer distances may not be focused as accurately.
Also note that it is not really crucial to have a lens perfectly focused, most of the time, a small error of a few points AFMA is not noticible.
The actual curve below shows this, there is little difference in sharpness at the peak of the curve, even if AF is off by +/- 3 points.