Don't miss this tutorial and
DOF calculator .
I'm also an s5is user and the main effect of using some zoom for portrait is to evenly distribute the DOF around your subject. At 50mm equivalent (8.33mm on our camera), you get a larger part of the DOF at the back, which is a problem when you want to blur the background. At 100mm (16.66 on our camera) from about 2 meters away it's already closer to 50/50 and you don't get much better by zooming more.
Since the maximum zoom of my 430ex flash is 105mm, I find it to be the sweet spot for my camera. Of course you would get a much shallower DOF using a real dSLR like a rebel xti or even better, a full frame dSLR like the 5D but you would also pay much more for those cameras...
Another way to get a shallow DOF is to shoot a face from up-close in macro mode but it is not always possible or practical to do so.