well that all depends on whether you want to make a custom rom like shinjitsu that is based on stock, or a fully built from source code rom like aosp, or cm. you can do all of the above in linux, but to build from source like we are doing for cm10, you need linux, also you can dual boot windows and linux like i do, one pc, and i just select which one i want at startup.
you can also help with cm10 if you like without even having to build, i cant build anything i just do some testing and work on debugging. then when i have an idea or know how to fix something i pass that on to someone that can build from source
and then there is what they call "cooking", that is when you take a rom, stock or otherwise and just modify it. that is just a matter of editing things, taking things out, putting new things back in, not always as simple as it sounds though, alot of times the really neat features require you to decompile things and tinker with the code, ive seen some of the things shinru has done and its quite amazing really. you wouldnt even imagine the time and effort he puts into that rom