I never turn the key unless dispatch can verify the weight.
It is simply not a good business practice to simply send a load offer indicating "Weight Unknown, Dimensions Unknown."
Moreover, serious ramifications may arise if dispatch responds to an O/O's inquiry regarding the freight with the answer, "I don't know...they just called for a van, or straight truck, or tractor". Why? Because it may cost the carrier the account when a truck arrives and it can not safely or legally carry the load.
When I sit for a while, I often contact local churches or Christian salvage warehouses to see if they have any hymnals, Bibles, Christian School materials, etc. that I can take home with me. Our church ships container loads of these materials (free of charge) to help local pastors in other countries.
If I load 500 lbs of used Bibles, and dispatch then asks me to consider a load offer, it is imperative that I know tht exact weight of the freight before I move toward the shipper's location.
Plain and simple...it just comes down to everyone being responsible for their part of the process.
I'll deliver the freight on time, I'll secure the freight properly, I'll conduct myself in a professional manner when dealing with the customers and I'll even brush my teeth twice a day.

That is my responsibility. I only ask that everyone else involved in the process have the same desire to do their part thoroughly and completely.
PS. I trained CSR's for a company and I made sure the warehouse every bit of information they needed to quickly process and ship the order; information gathered by asking the proper questions. If the customer could not provide the necessary information our CSR's would ask them to please call us back as soon as that information was available and we would gladly expedite their order.