Posts are installed after the fabric is properly placed and compacted in the soil. EvTec found that compaction before posting significantly improved effectiveness. Posting independently allows the contractor to space posts as they are needed - where water concentrates posts are spaced closer together possibly at 3' apart, and on legs with concentration 8' apart.
Steel or wood? Doesn't matter to a tommy™ installation. Wood posts are allowed or specified in many parts of the country, but as regulations and suppliers are now preparing silt fence to actually pond water and hold sediment, steel posts are likely to continue gaining presence.
Problem with wood stakes
If not sized appropriately, (actual 2x2) smaller sizes break while installing them. Properly sized wood stakes will perform acceptably, but their physical size (40 inches) prohibit achieving proper depth to support the horizontal load. Generally, stakes must lie as deep in the ground as the potential load above ground.
Wood Stakes may be less expensive than steel, but if the fence won't hold up it doesn't matter.
Providing 18" of sediment and storm water is a major difference and benefit of proper installation by slicing with the tommy™. The fabric doesn't work out and inferior posts are no longer adequate (as in pre-fabricated silt fence).
Steel posts are often the best option. Steel posts can be driven the proper depth into very hard clay and even rocky soils. Steel posts can support the 18" horizontal sediment load (when properly placed)... and can be recycled - often wood posts are not recyclable.