Well, I would have to say the first person who initiated the referral. I feel this is especially important in this environment if the first referrer is setting up, designing and maintaining the said site!
If it is just that they found a link to dreamhost on your site, or happen to find dreamhost on their own, and clicked on another user's referral link, then it's at the hands of who the last referrer is.
That is why I find it very important to sit with my clients in their office, or in mine, and help them purchase the site. Then I can see if there is another webid when I click purchase...I then reset it up with my web id. This has only happened twice.
So, that is my way of dealing with the rewards. If dreamhost would allow me to use my credit card on the purchase of more than one package...and give me the referral, that would be great. But, that's not how their referral works. Which I understand... That is the way reselling hosting works. The problem only arises when I have a client who doesn't have a credit card...and requests that i purchase it for them : (