People do find "good new things" by breaking rules.
So, I think you should know some of the "rules" that I use when developing filters for "general use".
Every single filter recommended in http://www.imagemagick.org/Usage/resize/#nicolas
is (pardon my math) C^0, that is, the filter is continuous. In particular, there is no jump at the limit of the window.
The reason for this is that, esp. when enlarging, jumps (at the limit of the window or elsewhere) will lead to very strong artifacts.
Actually, only two of the recommended filters are not C^1 (continuous with continuous gradient): EWA and tensor Triangle are the two exceptions, and they are only recommended for downsampling.
Indeed, if a filter fails to be C^1 (fails "enough"), you will see "ridges" and like artifacts when enlarging.
Two of the recommended ones are C^2 (EWA and tensor Spline).
Sigmoidization does not affect whether a filter is C^n or not.
Although some may say that I am rather picky, I do my best to produce filters that, at all times, are free of avoidable "bad artifacts".
If your image is very smooth with a nicely bell shaped color histogram, small evils are likely to be forgiven.
Unfortunately, Murphy is waiting around the corner with a pitchfork.
In addition, there are evils that can be forgiven when processing an audio signal with a filter that uses lots of data points (with many "taps"). Image resampling filters generally use few points in each direction. Generally no more than about 9 (the number for a typical 4-lobe method) when enlarging, and usually less. So, these small evils (like lack of continuity) generally hurt more in image resampling that in traditional signal processing, esp. when enlarging (since, when reducing, more pixels are grabbed and filtered).