A lot of the loads I hauled were drop and hook on the loading end. On the delivery end, customers normally want the product at a specific time. At time when their tanks are low enough to hold the product but before they run out. But rarely have I not had time to get where I'm going without problems. There are customers though that you do drop the loaded trailer. The main customers that each terminal services determine this.
In my personal experience hauling hazardous waste for Dupont was mostly drop and hook. Unfortunately that company (Sentinel Transportation) no longer does the chemical side, only oilfield and fuel. Hazmatinc.com took over those loads. Don't know how it goes right now.