You don't need to worry about the order of your script, but I suggest you have some plan if your script is 1000+ lines. If it's in `Update` or `LateUpdate`, then it will be called every frame. If it's `Awake` or `Start`, you could put it at the bottom, and it would still be called at the beginning. The only reason to worry about order is if you have a massive script and need to have an organized plan ( comments are great for this!!! :) ).
It really doesn't matter. It's conditional compilation so it generates different code in the end. It's like you have two versions of your script and depending for what platform you want to compile you exchange the script. It's a kind of preprocessor before the compiler even compiles the script.