....'Letting it happen' was a bit tongue in cheek, I do not believe anyone actually does that...
Hi Graham....this is precisely the approach that we see from absolute first time turners who proudly post their first piece of work on the forum gallery. I know for a fact that it happened to me and we see it from time to time with others. A lack of understanding of the tools and the way in which they can be used in experienced hands, tends to result in quite awkward shapes that don't 'flow'. Square sided bowls are a classic example of this. Thankfully, there's lots of patient guidance on this forum to help the newbie out.
Anyway....getting back to the main point. My design process works in two ways. firstly I get inspiration from something that I see, then I mull it over in my mind for days on end, sometimes weeks ore months. I find that the shape evolves in my mind, as does the process of turning it, and by the time that I mount the wood onto the lathe, I've got a pretty good idea of how it's going to turn out. There may be slight variations dictated by the piece of timber itself, especially by some beautiful feature of the grain that would be reduced to shavings on the floor unless the plan is adapted to accommodate it.
Les
p.s. ... incidentally, there was an excellent thread on this subject several months back, initiated by Mark Sanger. If you go the General Discussion area of the forum, then open page 10, you'll see it listed.....hopefully this link will take you there..
http://www.awgb.co.uk/awgbforum/index.php/topic,1795.0.html