Shutting down 1 (or 2) on a 747 (both Classic and 400) is usually done to prevent the taxy speed getting too extreme. Number 3 is the engine of choice as there are no brakes powered by the number 3 hydraullics.
My company has introduced single engine taxi in, but as there is a 3 minute cooldown required between de-selection of reverse and shutdown, it's only useful if you are expecting a longish taxi in.