Wileygirl,
I don't think any of us can say a certain approach, method, or technique of writing a work is right or wrong - it's what works for you (or me, or anyone else). I've tried several approaches, and right now I am rewriting my first draft and find myself writing sections, sometimes snippets of parts of chapters. Later I will determine what I need, and then what needs to go where. Writing is a journey and it doesn't matter how you get to your destination, as long as you get there.
As for having the ending, I for one like to know where I'm going and in my opinion, the ideal approach is to have A and Z, then fill in B . . . Y.
Jackster.