My usual advice is you want to incorporate when you either have (i) contracts with third parties, or (ii) other people working on the business with you.

It's a spectrum of risk, with coding alone with no customers on one end and a full fledged startup on the other. When you start signing contracts, you want the company to be on the hook for any breach of those contracts. And you'll probably want a separate bank account and a professional looking name on the signature line anyway. More importantly, if someone else is working with you, you need to make sure there's an entity that will own all the IP, and that ownership is clearly defined, with everyone subject to vesting to protect you from the co-founder walking away from the business.

disclaimer - I'm a startup lawyer but not your lawyer...