This has been an established part of Superman's characterization since his earliest comics. The implication being he had to "learn" to manage his strength from the time of his boyhood, and apply only the barest fraction of it to function in every day life, let alone to dealing with mortals in any type of physical situation. There are several instances of this being alluded to....
In the short pre-Crisis tale Martha's Story , by Samuel Hawkins, a dying Martha Kent is recalling various aspects of raising baby Clark. Several instances are referenced where his inhuman powers were showcased as him being a "handful", mostly due to his young age and resulting lack of control. The full short story can be read here:
Excerpts from it give detail....
"It would be nice to think that once we snatched Clark up out of that rocket, that everything was okay for him. But it wasn’t. Those first few months were rough. Being under our sun was quickly developing his powers, and adjusting to them was harder than you’d think. Suddenly, everything about him worked so much better that it was all happening too fast for him. We had to work very hard with him to slow him down. To teach him how to live at a human pace....
It continues [emphasis mine]...
"To say that Clark was a handful when he was a toddler would be an understatement. Most kids are, after all, but a child who hardly ever gets tired is something else all together. No naps for that boy. He was always on the go, hanging off the ceiling, or running around so fast that the wind he kicked up would pull things off the walls. Why, I didn’t think I would ever teach him to not use his super-speed in the house .....
I'm highlighting the next part to show how even from an early age, Clark's mental control was implied.
Of course, we would be kidding ourselves to believe that we could have ever made Clark do anything he didn’t want to do. No, him obeying us, allowing us to discipline him and parent him, always required his cooperation. He figured out that little fact pretty quickly, but he never took advantage of it. I guess his consent, unspoken though it was, was due to the good parenting Lara and Jor-El did in the time that they had him, and the fact that he was so smart, he somehow understood that he needed parents to teach him how to behave.
Of specific interest here is this part:
...He shot out of my lap and ran outside, and the next thing I knew, he had punched a hole in the little concrete pump house we used to have out back. Then he punched it again, and I could see that the whole thing was about to tumble down. I imagine it wasn’t the brightest thing I’ve ever done, but I ran up behind him, forgetting what someone as strong as him could do to me...I grabbed his arm before he could swing again, and he jerked it away from me so fast that I went down onto the ground. He didn’t mean to do it, but he flung me down like I was a rag doll. I looked up, and he was just standing there, this terribly angry look on his face, looking for all the world like he wanted to keep on hitting something. And with me being the closest thing at the time, I wasn’t all together sure that something wouldn’t be me....But then he saw me lying there, and saw what he had done. I’m pretty sure that was the first time he realized just what he could do to other people. And he just melted. He fell down on the ground beside me and sobbed and sobbed. "Not hurt Mommy," he kept saying. "Not hurt Mommy."The moment before, I’ll admit, I’d been afraid of him. After that, I never was again. He knew what he could do, but he was smart enough, and good enough, to never do it."
Another instance of Superman basically explaining that he had to spend "long hours in practice of pulling his punches" can be seen in a 1980s "recreation" of the original 1938 Action Comics story. You can see it here:
It should be noted that the Superman in this issue had power levels similar to that of the 1938 version; able to lift houses, jump an 1/8th of a mile and out-speed bullet trains, but nowhere near as powerful as even his Post-Crisis version which could actually fly and hold up tectonic plates.
A more recent example would be from the early 2000s Emperor Joker storyline. To nutshell this: Joker essentially steals Mr. Mxyzptlk's powers, becoming nigh-cosmic level and being able to manipulate reality. Consequently, he was also functionally un-killable. After turning giant and crushing Lois Lane [whom he had mind-wiped into being his Queen, Clark got pissed and literally plowed through the Joker's head in an attempt to stop him, as seen here....
But, he survived, of course, and made a joke about it here:
It's after this that Superman freely admits to himself that "I did hold back a bit".... even against a cosmic-level foe, simply because that's his nature. The implication here is that this control [and his vow to not kill] it's so ingrained in him that, even at that level, there is still always some form of restraint.
Even the animated series took note of this; it's the whole reason the "World Made of Cardboard" trope came into existence.
In one of the most spectacular episodes of Justice League: Unlimited, Superman became the codifier for this trope, because he gave it to Darkseid before delivering a sweet beat down on the God. Again, this is one of the weakest versions of Superman, so his "unleashed power" [which to me, logically, should have been closer to only 3-5%] only sent him flying through a few buildings and spilled some blood instead of knocking him totally off planet and shattering bone and innards. Still, it was a prime example of him literally saying how he "holds back" pretty much all the time. You can see the actual clip here:
So yes, he pretty much is always holding back in some degree, especially when dealing with normal humans.