If you're going to install an aftermarket cooler, then do it now when you first build it. Because installing one after the fact requires completely removing the motherboard from the case, being an 1155 socket.
Not entirely true. Some cases nowadays (even relatively cheap ones) have a cutout panel that gives you access to the back of the motherboard where you'd install the brackets. Having said that, I would suggest getting them all at the same time. Not that you'd even turn your computer on without a heatsink/cooler.
The case he purchased does have that cutout, by the way. And from experience, I would suggest screwing your MoBo in and using that to help with the installation of the standoffs and mounting bracket. Or maybe I just failed at that step when I installed mine!