Part of your confusion may be due to terminology--what is meant by manual white balance? On many digital cameras (including yours I think) there are two ways to set WB manually--one is to manually select a predetermined setting like "cloudy" (which may have an associated color temperature like 6000K) and the second is to perform a manual preset operation that involves the use of a gray or white card. Obviously, the first is simpler and the second is usually more precise. If you are selecting a predetermined setting, a good rule of thumb is to remember that most people will generally prefer a photo that has warm tones over one that has cool tones. So if you are debating between "daylight" and "cloudy," pick cloudy because it will result in a warmer tone photo. Likewise, "shade" will be warmer still. Of course, if you want cool tones in your photo, the advice is reversed.
The reason to do a WB preset is because the actual color temperature is seldom as easily predictable as just selecting "daylight" or "cloudy." The preset function in effect uses the camera's auto WB abilities, but with a very important difference. When you use the "auto" setting while taking a photo, the camera can be confused if the scene you are photographing has a strong color cast. The camera is trying to figure out what the lighting is like based on what it "sees" thorough the lens, but it doesn't know whether it is looking at something that is supposed to be red or blue or whatever. (For that reason, a few cameras have a second WB sensor on top of the camera that only looks at the color of the ambient light.) When you do a WB preset, you put a known neutral colored object (a white or gray card) in front of the lens (exposed to the same light source as your photographic subject) and ask the camera to compute an auto white balance that it will use until you make a further change. Since the camera's WB sensor can't be confused by the color of what it "sees," a WB preset is usually the most accurate way to set WB, assuming that you want perfectly neutral color. See your camera's manual for specific instructions on how to do a manual preset.
As others have noted, when taking landscape shots around dawn and dusk, you often want to have a very warm appearance to the photo to give the feeling of the light at that time of day. If you use a preset WB or set the WB to daylight, your photo will likely appear cooler in tone than you would like.
I second the comments made by others that you really should try one of the free or inexpensive RAW converters and shoot some photos in RAW mode. For many or most of your shots, the auto WB setting will be just fine. For those occasions when the auto setting is inappropriate, you simply move a slider in the RAW conversion program until the color suits your taste.