If you're great at something, you may find that you can do it faster mentally. However, rules and processes like showing your work will help you with more difficult problems you'll encounter later. Prepare yourself by learning now, before you're unable to slide by on talent.
Commentor austenallred explains at Hacker News what it it's like to take maths in university after coasting for years:
Of course, the maths got more complex, but I was good enough at doing it in my head that, at least for a long time, it never really mattered.
I thought it was because I just "got" maths, and the other kids were on a lower level. But as the maths grew in complexity, I fell behind. By the time we reached Calculus I was still doing most of it in my head, as I had never really learned to write it out on paper. And the complexity of the maths outgrew my capacity to visualise. I showed up to my AP calculus test without a calculator, partially because I was forgetful and partly for fun, and it wasn't until I got my score back (a failing 2 of 5) that it finally hit me: I was actually behind. In school. I was cocky enough that this was a slap in the face.
I had to start from scratch, and I'm still not sure if I've made up for a lot of that. I ended up in more creative fields, mostly because I felt inferior to those who had learned the rules and not been cocky douchebags like I had been in the beginning.
You may be able to coast on your talents for now, but your problems could get more challenging down the road. By then, you may not be able to learn the rules quickly enough to stay head — or even keep up — with the rest of your peers.