I'm getting reports from the "by server" version of the postfix mail stats script here PostConf | Spam Reports nightly via cron, and they detail the percentage of mail blocked by each RBL configured. (Nice script, btw.)
It very much seems that with the zen, barracuda, and spamcop RBLs, whichever one I list first blocks the majority of the total RBL-blocked spam.
To me this suggests Postfix actually queries the first RBL then moves to the others in sequence until a positive is returned, rather than querying all RBLs simultaneously as sometimes seems to be suggested. (Ie: "You'll get one DNS query per list you configure for each message checked.")
Perhaps slightly academic as I don't want/need more RBLs, but does anyone have better evidence either way? (Or, said another way, how do I log all the DNS lookups on my CentOS box ;-)