Depends upon the subject matter. If the lowering of the shutter speed will not cause unwanted motion blur, then always (without exception) decrease the shutter speed. Otherwise, you will have to increase the ISO in order to prevent blur caused by the movement of the elements within the scene.
ALWAYS, without exception, use the lowest possible ISO. Raising the ISO will always lead to a loss of sharpness, color, contrast, dynamic range, and an increase in noise. The noise will need to be removed via noise reduction software which further reduces the sharpness.
Other things to consider:
1) Never press the shutter button when using a tripod. When pressing the shutter, the camera can move slightly and end up causing some blur. Always use either a remote, a cable release, or if you don't have either, then you can just set your camera on a 2 or 10-second self timer.
2) Use mirror lock-up feature if your camera has it. The motion of the mirror slapping the top of the mirror box can induce some vibration which can cause blur in your images. This is especially true when using high magnification such as when doing macro work. I've tested several cameras that I've owned over the years, and without fail, the images with the mirror locked up were always sharper.