Eep. Never. I don't let anyone see the book until I've taken it as far as I can without outside feedback. That's partly because I don't have that many betas/CPs, and I don't want to waste their fresh eyes before I can best use them, but also because it would freeze me up. It's hugely helpful for me, as I draft, to know with certainty that no one will be reading this version.

THAT SAID: I have lots of friends who find it super useful to have a critique group that they can bring new chapters to, who can cheer them on, beg for new installments, and help them work out problems as they come up. So, if it works for you, that's great!