I'm planning on upgrading mine to 12 GB, so that later down the road if I want to upgrade to 16 GB, I only have to replace two of the modules and not all four. So in the long run it's more cost effective to make the jump to 12 now while you still have 2 open slots.
But if you don't think you'll ever use more than 8 GB before you replace the computer, save yourself the money and go with that.
If you believe 4GB ram prices will continue to drop then it might actually be cheaper to upgrade with 2x2 now and replace them all with 4GB rams later.
For instance, when I bought my iMac last November, 2x4GB was over $400 but 2x2GB was $110 (this at Crucial.com). I upgraded to 8GB with the 2x2 at the time, and if I wanted to go to 16GB now it would be $600 (4x4GB) for a total of $710. If I bought 2x4GB then and bought two more now to get 16GB, I would end up paying over $700.
Of course I'd have the benefit of 12GB instead of 8GB for the past 9 months, but I've never maxed out the 8GB yet.