Your choice of Hard and Soft closes depend upon where the prospect is above or below the power line. Using the wrong form of close will not give you results either above or below the line.
Your negotiation plan is a key element, especially when dealing with purchasing agents and those below the power line in the account. Relationships with junior people are more likely to be damaged if you attempt a hard close because the decision to issue a purchase order today is beyond their approval scope. Giving up margin too early and clearly not getting something in return – deviating from your negotiation plan – will end up leaving too much on the table.