As I understand it, it doesn't matter what you have your rewards set to, if someone uses your promo-code then that referral is automatically set to one-time rewards payment, regardless of whatever you have your rewards set to.
Here is a quote from the panel (the highlighting is mine);
[color=#0000CC]"The only downside is whatever discount (new! and/or feature bonus!) you give them comes out of your $97 one-time rewards payment (anybody who uses a promo code will be set as a one-time rewards, not recurring). But hey, that's only fair! It's basically a way for you to share some of your rewards with the people you're referring."[/color]
Web Hosting Reviews | DreamHost Promo Code