If you're selling a commodity product available on multiple websites, having a presence on coupon and cashback websites can help you capture some of the aggressively frugal customers. However, you need to be prepared to have the best deals if you want these customers.
On the other hand, if your product is only available on your website, you don't need to compete as much on price. In this case, coupons are more useful for generating a sense of urgency. If you offer coupons sparingly and with specific time limits, the frugal customers can feel compelled to order now before the price goes back up. However, you have to balance this against a perception than they can get a better price by waiting for the next coupon to appear. Use this opportunity to capture them into your e-mail marketing campaign (Sign up to get news and special coupons).