"The Mist" is another one where it really doesn't matter; some similar plot devices have led some people to connect it to the Dark Tower, but there's nothing explicitly linking the two. You could stick it near Bag of Bones
and the last twomain series books, since they all use some of the same (real-world) settings.
It's not that Insomnia
is inconsequential. It's very relevant to the Dark Tower series in theme and world-building. In fact, it's probably the single most important tie-in, though Hearts in Atlantis
is another strong contender for that title. But if you take it as absolute gospel, you're going to have expectations for the final book that won't be met.
The Wikipedia article isn't the best guide; a lot of it is people treating their own speculations as fact and putting stuff in the wrong section. The Regulators
honestly have nothing to do with the Dark Tower except a few pieces of fantasy language and world-building that King reused in rather different ways. But if you want to include it, by all means do so; the strong connections between the two are a kind of miniature version of the Dark Tower multiverse anyway.
...Now I want to reread all this stuff.