I coded professionally for more than 10 years without thinking about ergonomics, and without any problems. But then I started to feel pain in my arms, and it took a lot of effort to get it under control. In the end, what helped me the most was using a break program (to remind me to let go of the keyboard, and to do stretches), in combination with an ergonomic keyboard and mouse.

I've written more details about it here:

https://henrikwarne.com/2012/02/18/how-i-beat-rsi/