Whaaaat, I feel short changed here, my tanks generally opened with a combo in defensive stance and pretty much never went back. if I go back to January (We killed it earlier, but it wasn't until here that we started using a warrior pull and holmganging a few heat/spears), our warrior used just shy of 20 GCDs in defiance (spread across 3 occasions) throughout the entire A12S encounter. In the same kill our DRK used Grit twice, once just after the intermission to stop me pulling agro during the first stasis and again just after the second stasis as a mini cooldown.
On the flipside of the coin, myself and my co healing SCH used Cleric stance 9 times each, I had a 15% total uptime and my SCH was at 90% =o
This is something that really varies from group to group, I push a ton of HPS to allow my tanks and co healer to spend most of the fight DPSing as hard as they can. Other groups may have a more balanced approach to this (as Evy's demonstrates).