I will definitely leave a few letters. I will probably send them the day I CTB or schedule emails for after I CTB.
I'm going to make sure I'm thorough. I don't really know who I would send them to honestly. It's going to be a long time before I actually CTB so I don't know who will be significant in my life in those moments. I think leaving a letter just depends on your situation. If you're not close to your family or don't have anyone you really care much about, why give an explanation? Unless you really want it made clear.
Ironically, I have plenty of people I'm angry at and would want to leave letters for them too. They've hurt me bad enough I know that won't change. And that's not even for vengeance, more just a final "goodbye, and I still hate you even in death."