Siping is done to Blizzak tires and is part of the reason why they are such great snow tires (and last a very short time in anything but snow). If you touch a Blizzak, you will see that the rubber compound is very soft/porous and also has many fine cuts/sipes.

I'm not sure what effect siping would have on a standard all-season tire, but I would imagine that they would grip a bit better in the snow and reduce tread life somewhat.

I think in general most modern tires perform so well that siping by hand is not cost effective.