Spoilers in this section
I've never needed something to be canon to ship it.
From my first readthrough, Jiang Cheng was my favorite, and I loved Nie Huaisang for many reasons, but I must admit I was not shipping them (if anything, I thought Jiang Cheng's obsession with Wei Wuxian was very shippable). It was only after, that I decided I would like Jiang Cheng to not be sad and lonely forever, that I decided I wanted to ship him with someone I could see him being happy with.
And I settled very quickly on Nie Huaisang.
The very first reason is their friendship from their teenage years. I'm a sucker for friends to lovers. They knew each other, and liked each other, before tragedy tore their lives apart. Nie Huaisang knew Jiang Cheng before he was the feared Sandu Shengshou, and Jiang Cheng knew Nie Huaisang before he put on the Yiwen Sanbuzhi (Headshaker) mask for good (yes, he was already playing around with this since they were 15, but not to the extreme it got to after Nie Mingjue's death). I liked to think they remembered who they were back then, when everyone else forgot... and possibly, through remembering the other, they might remember themselves.
There is another very important component of the teenage friendship scenario, and that is the fact that Wei Wuxian left the Cloud Recesses after three months, while Jiang Cheng stayed. The book doesn't say exactly how long that stay was, but it's safe it say it was up to a year. So yes, it was the three of them at first, but for most of that time, it was Jiang Cheng and Nie Huaisang, together. We don't see it, because it's Wei Wuxian we follow, but it's not unreasonable to think Jiang Cheng and Nie Huaisang become even closer friends during that time. In fic, it's certainly where the seeds of their relationship are often planted, or where it actually shifts to romance, even if initially it's just experimenting.