It really depends on a constellation of factors. If you are planning on pursuing investment, there's a traditional structure most angels/VCs want to see. If the product has a small number of customers/large margin, you need to swing the team toward sales/customer relations. If it's a large number of customers/small margins, you need to swing toward marketing. If the product is unique in its niche, you need a smaller dev team than if it is in a crowded market.
You need to cover all the functional areas, but your choice of who provides input on an internal vs. external basis is subject to the company's product attributes and market landscape.
You also need to shape the team around the strengths and weaknesses of founders. At Apple, the first person to have the title of president was an operations/production person because other key bases were covered, and the founders recognized that their Achilles heel was the experience of managing the shift from product development to building product in quantity and getting it out the door.
By design and due to the availability of key players, the early structure is rarely correct for a more mature company, so plan to reassess functions and team members in 18-24 months. This shift in team mix is one many startups recognize and acknowledge by setting up stock/option/warrants to fully vest quickly.