It depends on the company/department culture and belief system. If the company believes that it has much to gain from a flat organization and learn from each other, then, go flat. If the company feels, that structure is more useful for customers to be able to feel they're getting their monies worth when their issues gets properly escalated, then create a tiered structure, leve, 1, 2, 3...etc. This is true for QA, Engineering or Services.