At this point, after much deliberation, I'm converted to the factors being for planning purposes only.
We plan conservatively. We get a more direct routing, maybe a more favourable level, and save a bit of fuel. We're a little heavier, but we will be close to our planned weight and the conditions should be similar to those forecast. If it all changes horrendously for the worst, or we've had an abnormal situation, we revise with judgement. I take a while... But hopefully I get it in the end.
As I think safetypee spoke of in their previous thread (which was very informative - in case my previous post didn't seem to appreciate your input SP), we're judged over the course of months on a decision made in a dynamic situation.
Thanks to all who've contributed and helped clarify this for me. Without wanting to sound too sentimental: Merry Christmas!