Try it anyway. I just put something from the New eBooks listing into my shopping cart and even though the coupon is technically expired, it still deducted the price.
Mind you, I haven't clicked through to purchase yet, but September's 50% off coupon kept working for about 2 days after official expiry and if you're worried about getting dinged, FW seem to be making a habit of doing this every couple of months and no doubt they'll have holiday sales.
ETA: Just so you don't get the impression I confused the standard 15% new book discount, it was Greg Bear's
The Forge of God, regularly 9.99, new book discount to 8.49, and 50% off discount to $4.25 in the shopping cart.