(Not a lawyer) I remember being told a rule of thumb that you should be a separate legal entity before "first contact" with a customer because that's generally where most of the risk of legal liability starts ramping up.
That's one approach and a defensible one but I think that might hamstring efforts to build a company to the point where sponsorship is an option. Another approach is providing services or products for a fee. But this is really a grey area and risks must be managed against the ability to be an entrepreneur and create valuable business and technology.
