I use PostGrey at the moment, and that comes with a file called "postgrey_whitelist_clients" that lists specific domains that should be automatically whitelisted, because they either don't retry at all, have weird patterns, long delays, or big pools of sending addresses that make normal greylisting problematic.
You can, of course, tweak the list yourself, if you find specific problematic senders. Postfix does now also include a tool called PostGrey that can do things like RBL checks before a message even hits the SMTP server, and checks similar to PostGrey. That can, apparently, reduce the load by getting rid of a lot of problems before a message even gets as far as being fed into SpamAssassin. I intend to experiment with that, however since my present experience is with postgrey, that's what I'll be using here.