I never had any tearing on my old 2x 5870s when using 1 native displayport and 2 DVI. But, I do get tearing with 2x 7950s when using 1 mini displayport to displayport, one DVI, and one HDMI. I get the feeling that you can just luck out with a particular card/port/monitor combination that doesn't get tearing but that Haldi is correct that the only way to be certain that you won't get tearing is to use all displayports, which would require a non-reference card.
I've found that it's impossible for me to eliminate tearing altogether with my current 2x 7950 setup. But, I find that different games respond differently to the use of v-sync. Some games will tear on the center monitor unless I turn v-sync on. Then, tearing will move to my side monitors where it's not as noticeable. On the other hand, other games will having tearing on the side screens with v-sync off but will move tearing to the center screen when vsync is on.
So, it takes some fiddling to get "optimal" tearing. I'm really hopeful AMD will work on something similar to NVidia's "adaptive v-sync" technology that eliminates multimonitor tearing. HardOCP did a good write-up on it a while back:
http://www.hardocp.com/article/2012/04/16/nvidia_adaptive_vsync_technology_review/