This is a common fear among many nano users. I also spent a few years worrying about how close my herb was to the heater and if anything would land on the tip of the heater shield. I eventually threw caution to the wind and just stopped caring about it. I started packing bowls that would sometimes snug up against the heater and have never charred the herb. Now, my nano is a very moderate unit. Some run hotter, some cooler. Particularly if you need to use a converter of some kind because of being outside the US. I vape herb at a touch above a 6 on the dial.
Of course if you turned it way up beyond normal vaping ranges it would char but I doubt mine would burn. One time, I got a little too faded and left a packed bowl on the heater. I didn't realize it until the next day. When I examined the pack, the herb was dark and dry, but it did not char or burn. Thank heavens. No, I didn't finish it.
And the herb pack will fuse into a little puck once the heat starts flowing so you don't get flakes of herb landing on the heater much. Particularly when you're vaping through water since the nano is usually up-side down.