1. use education or corporate store, it's slightly cheaper.
2. If you are like me who prefer logitech to apple magic mouse, you can exchange for trackpad instead.
3. It's more difficult to DIY anything with the new iMac except RAM. So BTO everything from apple store such as processor, graphics cards and SSD (don't buy RAM from apple).
4. If you plan to get other apple accessories such as airport extreme, do buy them at the same time and they will have extended coverage when you apply apple care to your imac.