How do you make a item line level (under the items tab) field mandatory for ONLY on Sales orders? We need the field to show on other records but if we leave it mandatory we cannot get vendor bills via EDI as they don't have that information as it is internal and it errors out because it will not create the new record without it. Any help would be greatly appreciated.

Make it as mandatory by using Workflow (which applied to the Sales Order)

Won't the workflow cause the sales order to be slower since it would be on every item it has to check?