We send our review requests for each order out in a single email (using the multi review email template). And we give a coupon for when customers write reviews. But at the moment, for every review they write in that single review request email, the customer will get a coupon. So if the original order was for 5 products, they write 5 reviews then they will be emailed 5 separate coupons, which they can then use on 5 subsequent orders.
I think it'd be better if there was the option to just send one coupon per order (and not for every review in each order).