No hard-and-fast rules, only deep-rooted conventions:
In financial statements of listed companies, stick to millions.
In other financial writing, switch between millions and billions as appropriate (avoid things like 0.001 billion)

(Financial statements of listed Japanese companies use units of 100 million. Using billions will often compel three decimal places, which looks odd especially with small numbers (i.e, 10 million) and is less readily grasped by the reader, while flip-flopping between billions and millions is frowned upon by issuers. )