MRP & Dates

I am trying to figure out the dates that drive MRP. For example we have a line type W item that automatically generates a work order when the sales order is entered. My understanding is if the request date on the SO is 02/28/11and if the Lead time for the part is 10 days then the Start date of the WO is 02/14/11 (02/28/11 - 10 working days). Is that correct? Is the WO start date=SO request date(SDDRQJ) - Lead time of the part? Or is WO start date=SO Pick date(SDPDDJ) - Lead time of the part?
Is the Request date for a purchased part in the BOM for the manufactured part=Start date of the Work order?