Do you know why they would recommend climbing out at VTOSS instead? Does it give a better ROC when you take the acceleration into account? I would assume that this would only be valid up to a certain distance..
Yes, it's for obstacle clearance, rather than obtaining best rate of climb. For example, the profile for the type I fly requires the aircraft to be flown at Vtoss (30 kts) up to 200' above the takeoff surface, then an acceleration to Vy up to 1,000'