The reference to the keyboard controller for PORTA in Mapping the Atari is for the mini-keyboards like the CX-50 that plug into the joystick port, not the main keyboard on the computer. They're pretty neat devices, by the way; you can reliably detect two simultaneously pressed keys on them.
There are other bits in SKSTAT besides the key down bit, so checking against 255 is a bit unsafe. At the very least, make sure you write to SKRES ($D20A) at least once at startup to make sure that the keyboard and serial error bits are reset. You'll still have to put up with the Shift key bit. If you're using assembly, there's no reason not to test only the key down bit.
If you're going to disable the keyboard IRQ, consider whether you want the break IRQ either, or any IRQs at all. A viable option is to mask all interrupts on the CPU but leave them enabled on POKEY; this still allows your main loop to periodically check for key downs without missing any.