I use the grates that I need for each cook. If I’m not using them, they’re not in the grill.
Ideally, I’ll heat soak for low and slow cooks. In my hands, that means letting the grill sit at my target temperature for about an hour. This isn’t time wasted, as what I usually do for cooks is light the grill, set the vents for the temperature I want, then do food prep. By the time I’m done with food prep, I’m at least pretty close to being done with the heat soaking time.
Having said that, I did a rib cook recently where I didn’t have time to heat soak, and the ribs still turned out fantastic. I did light the grill first, and then prepped the ribs, but it was only 30 minutes or so of heat soaking.
You can hang the unused grates off the hooks on the side of a KK grill. I built a storage unit out of 2x construction lumber and 1x pine boards from Home Depot.