The soft focus feature can be tricky (and I wouldn't be experimenting at a wedding unless you're just a tag-a-long or something), I actually hardly ever use it, the lens is very sharp and capable with the dial turned to zero.
However, if you want to play with it the dial can be set anywhere between the notches don't feel like you have to stick to the 1 or 2 setting. The soft-focus dial will have no effect if you go above f/4 (the wider the aperature, the stronger the effect).
Here are some examples of what the effect will look like:
No soft-focus
http://www.squawkboxdesigns.com/135SF/135SF-.jpg
slight soft-focus (at 1/2 or 1)
http://www.squawkboxdesigns.com/135SF/135SF-7440.jpg
Full blown soft-focus (at 2) (just for fun)
http://www.squawkboxdesigns.com/135SF/135SF-7459.jpg