LTL: If an incorrect weight or class is used and a shipment is reweighed or re-classed by the carrier, those costs may be passed on to you. Reweighing may also delay delivery.
FTL: As long as you have ordered a Full, Dedicated truck, estimating the weight is fine as long as the shipper can be sure that the load does not passes the trucks maximum weight limit. (Usually 44,000 Lbs. for a Van and 48,000 Lbs. for a Flatbed)