I guess it all depends on how you work and write. My studio and keyboards are setup so that as I am composing I am also recording and mixing. I can envision what the final product will sound like because I'm mixing as I'm composing. Most often once I've finished composing the song its only a matter of adding some live instrumentation and doing small tweaks in the mixing process to be finalized. This process works great for me but presumes you can also engineer and produce your own music.

Many film composers and artists I know who aren't engineers have their own personal engineer on call so that when the creative flow strikes, they can compose, record, and mix without any down time in the creative process. Time is always a factor when the creative process is happening. An idea that isn't laid down quickly can often be gone forever.