For me, editing while still writing that first draft leads to frustration, and then feeling blocked entirely. It's why I have yet to complete Nano, because I just can't help myself from rereading what I've written, followed by cringing.
Editing, as in looking at the overall narrative flow of the story, making plots hang together and have all the elements required introduced at the right time, giving characters the right level of progression, managing the pacing to fit the point in the narrative arc... this stuff I can't do until it's all finished. The ending might require you rewrite the start, which requires you rewrite the middle, which requires you rewrite the ending... if you do it once you have a view of the whole then you might be able to avoid this.
In the end, all that matters is that you have a coherent, compelling story. Doesn't matter how it was put together.