Is it a good practice if I always use the highest shutter speed for regular shooting? I mean, in the day, with good lighting, where I don't need any special effects like trailing? I am thinking this is the best solution, since there won't be any shake from the hand whatsoever. Is this true?
It depends on what you mean by "highest".
If you have enough light, then the first thing you should do is to reduce the ISO setting to the minimum, so that you can get as much light as possible on the sensor1. Lower ISO means less noise, more dynamics.
If there is still enough light, then close the diaphragm a bit compared to its maximum possible aperture (unless you need a shallow depth of field). Most lenses have their optimal quality around 2 stops below their maximum aperture (e.g. an F/3.5 lens is usually best around F/7). It's just a rule of thumb, it obviously depends on the lens). Don't close it too much, the image gets blurry due to diffraction on large F-numbers.
Getting a fast shutter speed is good, but if you are not shaking your camera too much and the subject is not a racing car, you won't see any difference between "fast shutter speed" and "even faster shutter speed". A typical heuristic is to use 1/"focal length", e.g. 1/50s for a 50mm lens. But shooting at 1/1000 or 1/4000 for the same 50mm will not make any visible difference.
Also, as DetlevCM points out in comments, the best image is not always the sharpest: if the subject moves, then a bit of motion blur can be part of the composition and reflect the movement while freezing the movement completely with a fast shutter speed will give the image an artificial taste (DetlevCM already gave the example of a helicopter, but the same applies in many other conditions where the subject is moving fast).
- Strictly speaking, decreasing ISO does not automatically mean more light on the sensor, but in most cases it ends up being the case.
No. If you have a faster shutter speed, you must be either increasing the aperture or the ISO to compensate. Both of those have effects on your photos, which may or may not be what you want: for example you may not want to shoot with your lens wide open, either because you want a greater depth of field or because you know your lens isn't sharp wide open.
You should base your decisions for shutter speed, aperture, and ISO, on your artistic needs, so that you achieve a correct exposure, the amount of bokeh/sharpness you desire, and low noise. See What is the "exposure triangle"?
A high shutter speed will
- help to reduce shake, but it makes no difference after a certain point (typically cited as "1/focal length");
- but it also freezes motion, which may or may not be what you want (try shooting a fountain with different speeds),
- and it means you either need high ISO (noisy), or a wide aperture, which means shallow DoF, which is, again, a source for sharpness problems if your focus isn't spot on (or you want more than a small sliver in focus).
In general, the answer is "no" for the reasons explained in detail in the other answers given. In the typical situation the main focus should be on aperture as Rafael explains. But there are situations where the shutter speed should have priority. If you take pictures of fast moving objects like birds in flight, or you are moving fast yourself, e.g. you want to capture a scene from a plane just before landing or just after take-off, then a fast shutter speed is needed. In these situations it's easy to underestimate just how fast the shutter speed should be, the nature of the objects you are trying to shoot often doesn't allow you to correct mistakes by changing the setting and to try again. Then with one chance to get it right, you need a relatively large margin of error, so the shutter speed should be taken to be above a reasonable high end estimate.
Example: you are in plane and just before landing you see a nice view. Suppose you estimate that the plane could be traveling at 100 m/s and the objects you see could be 500 meters away. This means that the angular velocity is (100 m/s)/ (500 m) = 0.5 radians/second. If your camera has a typical crop sensor and the focal length of your lens is 50 mm, then one pixel will capture a field of view of about 10^(-4) radians. This means given your assumptions, above an exposure time of 10^(-4) radians/(0.5 radians/second) = 1/5000 of a second you would start to see motional blurring. This 1/5000 of a second is probably a lot faster than what your gut feeling would have suggested to you from just looking out the window.
Since we all desire that our image is a winner, we want high acuity. This is an aperture that is (as a rule of thumb) 2 f/stops down from max. Thus the best ISO and shutter speed setting for general usage, causes this to happen. Also, a rule of thumb that is useful -- the minimum shutter speed to use is 1 over ISO. Thus is your setting is 100 ISO then 1/100 of a second. If 400 ISO is set then 1/400. This is actually called the sunny 16 rule of thumb. On a bright sunlit day, you set the aperture at f/16 and the shutter speed at 1/ISO.
Is it a good practice if I always use the highest shutter speed for regular shooting?
It is not a good practice always doing the same thing. It is not a recipe.
In my humble opinion I rarely think of the shutter speed first. I think first about the aperture, because, in most cases, the DOF has more weight on a composition than the speed.
In second place I think of the ISO. Do I have enough light? or I need to force it a bit.
The shutter speed is than just a consequence of the 2 decisions to have a wide aperture or smaller one and the selected ISO.
If you are using a flash: no it is not that important either.
So I set my camera on Av... I never use the other modes.
No. I leave mine on 1/125 because I was told 45 years ago that's fast enough to deal with any movement of the camera and stop any non-speedy motion of the subject. That gives me more depth of field. Depth of field compensates for the other kind of motion a subject can make, which is not addressed by shutter speed: moving closer to or farther from the lens.