Anyone who has done a significant amount of interior or exterior painting knows preparation for the application of paint will make a huge difference in the quality and appearance of a paint job. Every crack or gap that gets filled is one more thing the eye will not be drawn to, when the job is completed. In the end, it is as much what is not seen as it is what is seen.
Note, for example, painted trim and molding inside many homes. Some have had the gap between the molding and the wall caulked. Others have not. Ones that have not been caulked have many dark lines the eye is drawn to. It really stands out. Merely filling that gap with white caulk, even without paint (e.g. applying just enough caulk to fill the gap and wiping most off with a damp rag), makes a huge difference. It's no different on an exterior wall.
In addition to the appearance, caulked exterior walls have another advantage over walls without it -the caulking reduces areas where moisture can get behind trim and siding and damage even a good paint job.
In cold weather, water can freeze in cracks and cause them to expand more. Of course, the water can be absorbed by the wood and freeze too, causing damage. Clearly, caulking well, with good caulk is a worthwhile investment, in time and materials.
One of the big problems I've noticed is, people, including pro's, use their fingers to spread caulk. Because of this, they leave smooth spots on rough surfaces, like cedar or other boards. I even see it on Hardiplank and other man made products. Like the shadows your eyes are drawn to, your eye is drawn to the out of place smooth spots on the rough surface.
There is one more problem regarding the application of caulk to rough surfaces - it's like dragging your finger over sandpaper. By the end of the day, every finger is raw, and your thumbs too.
To avoid the problem, I tried cotton gloves moistened in water. It worked. Later, I began cutting the fingers off the gloves and carrying them with me, so I could turn them and replace them, as they wore out. That worked too. However, the problem of leaving a smooth surface remained.
On a whim, while working a job I knew would use at least two cases of caulk, I grabbed a couple chip brushes (those white, disposable brushes you pick up at places like Harbor Freight) from my van. I cut them down so only half of the bristles remained. This left the bristles stiff enough to work the caulk into cracks and things, but still flexible enough to follow the wood pattern.
Using these brushes to apply caulk, I was able to fill gaps, but no longer had to worry about the smooth spots, on a rough surface. Of course, my fingers no longer got worked raw either.
Using the brushes, I just lay on the caulk, getting as much caulk into the hole or gap as I can, then I feather it out using the stiff bristled brush.
I found it helps to keep a partial cup of water with me to freshen the brush, when it starts gumming up.