My guild started a Heroic Tank raid and in less than an hour and a half, we were 25% into P3 and only 20 members had participated in the raid at that point.
I was driving and so missed P1 and P2. It happens but as time goes by I think it will be increasingly difficult to get hits on all Phases.
Currently, my guild launches tank at 11:00 am and the first three phases are a free for all.
P4 doesn't open until 24 hours after the raid is launched.