Sales Price related

For item with variants, can we setup one generic sales price for all variants ?
YES, just leave variant code column blank

Which date on source document is used to select applicable price lines ?
See codeunit 7000 Sales Price Calc. Mgt. func SalesHeaderStartDate:
IF "Document Type" IN ["Document Type"::Invoice,"Document Type"::"Credit Memo"] THEN BEGIN
    DateCaption := FIELDCAPTION("Posting Date");
    EXIT("Posting Date")
  END ELSE BEGIN
    DateCaption := FIELDCAPTION("Order Date");
    EXIT("Order Date");
  END;