As a practical matter, I don't think the species of wood matters nearly as much as how well it was seasoned and fitted, and cared for in use. Mark makes an excellent point about the role of gravity in storage. A lot of guns in the past were openly displayed in a horizontal gun rack, so individual guns will vary in how much oil is soaked in the head.
I have only been fooling with Smiths for about 15 years, so don't have as much experience as many. My impression is that cracked stocks are not as common in Smiths as some people think. I have actually seen very few. My beater hammer gun, made in 1901, needed some work. But, I have a 1946 16 ga. Featherweight that I have shot more than any other, and it has not issues. Of course, it is a Marlin era gun, stocked in Black walnut. I think Michael McIntosh did Smiths a disservice in his book Best Guns in this regard.
Also cracks may first become visible behind the locks, but they start in the head. If you can get the oil out of the head without damaging the finish, that's great. I'm a little skeptical about how to do that with cat litter, but if it works, that's great. I may may try applying acetone on one after masking off the finish to see if I can dry out one I'm about to refinish. Great thread.