Some tie-ins are direct continuations of subplots from earlier in a character's own series. So to fully 'align' them, you'd need to have read, say, the last six months of 'Black Cat'. That's the sneaky part—they're not just aligning with the omnibus; they're aligning with their own book's ongoing narrative, using the event as a catalyst or interruption.
Reading orders are a trap designed to make you buy more comics. The 'King in Black' omnibus is a self-contained story. Does it reference older stuff? Yes. Do you need to understand every reference to enjoy Silver Surfer fighting a symbiote dragon? No. It fits right into your afternoon if you have six hours to kill.
Why is everyone so obsessed with reading orders? Just grab the books and dive in! Sure, you might be a little confused, but that's half the fun with comics. The 'King in Black' omnibus has a recap page. That's good enough for me. These events are meant to be huge, chaotic, and over-the-top. Embrace the chaos!
Important note: The 'King in Black: Return of the Valkyries' mini-series is collected in there, and it actually runs concurrently with the early issues of the main event. It's not a side story; it's essential for what happens to Jane Foster. So when you hit that point in the omni, maybe read those issues together for cohesion.