Yes, I would think deposits and open or outstanding sales orders should be taken into consideration, especially if it is to be used as a tool for determining if a new order will put a customer over their credit limit.  
 
This is more critical when doing specialty items as opposed to "stock" items.  If the particular product can ONLY be sold to one particular customer, then open sales orders are critical to consider.  If I can sell the same item to several customers, it is less critical to track sales orders, just open invoices/deposits.
 
As in everything, a little intelligence has to be put into it with regard to knowing how a customer pays.  Some invoices may clear by the time the next order is to begin.