One senior in-house lawyer I know likes to get invoices in draft form. His defense of that practice is that once a firm sends a bill in final form, any write offs or changes cause internal problems for the firm. Secondarily, he believes that the draft reminds them that a bill is subject to law department review and scrutiny.
But his two-step process takes more internal time for it requires a double review (See my post of March 2, 2008: bill review with 25 references; and April 13, 2009: signing off on bill review with 6 references.). Far better to get final-form bills, many of which are approved as submitted, and let law firms deal with those few bills sent back marked down or corrected (if the firm even wants to submit a revised bill).