You only really need the "early" microcode loading procedure (the one where you pass it in the bootloader) if you have a microcode problem that is so broken the system won't boot with it unfixed (...or you're toggling feature bits off and the kernel will break if you originally booted with those bits and then they go missing - e.g. the broken TSX implementations on Haswell/Broadwell).
Otherwise, just loading the microcode update once you get into the running OS is fine. I've never actually had a system that needed to use the former, but I've certainly used the latter.