You can definitely do the install w/o a fastboot cable, but you do need to have a working fastboot driver on your machine. The KFHD's stock bootloader will check for fastboot commands for a short period of time during the boot.
There are other methods as well to get the partitions setup for this, however, in the case of disaster having a working fastboot setup will be required to restore.
Does a factory/fastboot cable work on the 8.9? Some say to buy it, and others say that it only works for the Kindle fire 2 and 7" and that there is no need for it. If it is needed, is there a specific cable that you recommend? Thanks