Depending on the manufacturer it will make some difference on the performance of the window. First thing to do is get a list of the overstock windows from whom ever your dealing with. They should all have some type of inventory list on hand for contractors and their own books. Have them give you a quick tutorial on how to read the list and know what the information means. The standard home owner may not understand the verbage used in the industry. Ounce you've compiled your lists of windows try and figure them into your remodeling plans. Ounce you've figured out what will work, where. You can negotiate a price for the windows. There are also liquidation stores that specialize in the auction of these over stocked windows for many companies. Let us know what brands you are looking at and post back.