AFAIK, there is no policy, official or otherwise, against one site = one IP.
IP allocations are supposed to be justified. But really what is dicouraged is allocating IPs that won't be used.
"Uh, I want 1,000 (or 10,000, or 1,000,000) IPs because I think that's how many I'm going to need next year."
Not unless you can show a history of that kind of need.
If you look in the small print at ARIN (the agency in charge of IP allocations in the Americas and Sub-Saharan Africa) I think you'll find there's even a policy for taking-back IPs. But the criteria is that they have gone unused.
FWIW, my (VPS) hosting provider charges $2 per IP, one-time (no monthly fee). So this need not be horribly expensive. I have one IP for every site. I do combine alias domains for the same site on one IP.
If for some reason you want to support HTTP <1.1 browsers, and want redirects to the primary site to be handled properly, it takes 2 IPs per site. (One for the primary, one for all aliases.) After looking at the browser stats, I decided that was overkill.
Here's one negative to having a unique IP address: you will be subjected to a higher volume of attacks. Some atackers scan through a list of domain names, others through blocks of IP addresses. Those that go through blocks of IP addresses will only go to the first site (usually a dummy) on a virtual host.