In Ranked, when I see the enemy has a DC early on, I instinctively don't push the KO immediately. I'm a gentleman like that. But I am no fool, thus I try to maintain tight control of the battle. Once a minute or two passed, I go all in again and it usually results in a KO, as of course in S+ lobbies having one player too few means to lose. After the aforementioned time span, I just think it's fair to take advantage, as Nintendo will most likely take the DC into account then.
Of course it's not always possible to be fair: When the enemy fights back like crazy and does serious attempts to KO us first, you're gonna have to wipe them and end it. Everyone has something at risk in ranked and as pointed out by Gameboy224, most people will use any chance they can get to grab that easy win.
But on the other hand, there's exceptions to that. Here's a nice story that once happened to me in Rainmaker:
We were on Pumptrack and a mad battle broke lose around the rainmaker in mid. I'd punch two of the enemies with my roller, another guy with a Splattershot takes two of our team aswell. In the same second as he took my teammates down, I hear the remaining lad despawn next to me. The distraction resulted in me getting splatted. We're wiped, enemy has two on field and the Rainmaker barrier was burst. While we respawn simultaneously, they already sped towards our volcano and are about to dunk. Suddenly I see the dude who got the Rainmaker to 2 break his push, speed to the ground and jump with it into the abyss. He stopped to give us a chance to have our DC being taken into account. A great dude.
We then fought a brave battle until 1:30 left. Then my two mates were down again and they were close to our volcano. I returned the favor and stood still as I granted them the KO. The DC was taken into account and we lost nothing but had a great and fair battle. I've never experienced so much sportsmanship in one game of Splatoon before.