Just found a note:
You can usually flash the radio separately because ROMs dont typically include them (or if they do, its an option in Aroma). Radios are therefore not usually affected by Rom flash.
Kernels on the other hand, are typically included with the ROM so if you want to use a different one, you need to flash it after the rom. Usually your fine flashing them together without rebooting in between. HOwever, sometimes it can be quirky so I usually boot into the rom and then boot back into recovery and flash a kernel.
The android setup (account stuff) is indepenedent of the kernel so do that whenever