If it was a coupon that is linked to a PC # that starts with 9, it's an internet discount and AFAIK, does not require the physical coupon. With these, I've never had to show a coupon (though I keep a printout, just in case) -- in the few cases where I've been asked for a coupon, I just mention that it was an internet special made at the time of booking, and that's been sufficient.

If the PC is anything else, I'm pretty sure you need to turn it in -- such as the $25 off "customer service" coupons, for example. Though in practice, I think the return agent don't always pay attention to it (probably because many of the coupons fall in the aforementioned "internet" category anyway).