When in the "buying a new house" process do you start packing?

Our offer was accepted on our new house a few weeks ago but we haven't exchanged any contracts yet. The average turn around time in the UK is 8-12 weeks because there is so much to be done between offer and exchange - agreeing the fixtures and fittings, searches, surveys, mortgage stuff etc. Once you have exchanged contracts, you usually get a week to organise yourself before the completion date - that's move day to you and me!

At what point would you start packing? I've got so.much.stuff to go through and the best thing would be to get started right away. But there's also so much that could go wrong, making our entire sale/purchase fall through. I know this is silly but I feel like I'm tempting fate if I start now!!

What would YOU do?