IS has a switch for a number of reasons. First being if you are using a tripod, if you have the camera rigidly mounted there is no need for IS and it will actually add vibrations. Another reason is when you are using high shutter speeds, there is no benefit to using IS, and having it on just drains your batteries quicker. Or sometimes people want some blur in their image to achieve the desired look they are going for.

Most cases I use autofocus and it works fine, but if I'm photographing a still object and want to make sure the focus is exactly where I want it I will use manual. The trick to manual focus is to use Live View at 10x so you can see better when the image is perfectly in focus.