Ask HN: When to write the full business plan?

I was discussing my early-stage startup project with an old Oracle exec today (who now may become an angel). This informal conversation/presentation was supported with screenshots and a 9 slide PowerPoint (note: See ‘Guy Kawasaki’s 10/20/30 Rule of PowerPoint’ http://www.youtube.com/watch?v=liQLdRk0Ziw if you’re interested in hearing what a VC likes in a .pptx during pitches). During the questions towards the end of the meeting, I was asked if we had already created a business plan.

I’ve written four or five full plans in the past at the hatching stages, but differently, they weren't for tech startups.

My feeling is that, at our stage, we should be spending the time developing our product (i.e. seeing what it can become/how it will evolve from iterations) as priority over writing a business plan. Possibly some financial projections could benefit our future meetings with angels, but even these forecasts would be completely abstract.

Can I ask your advice, when should a full business plan be written?