I don't have an informed answer, but a couple observations. First, mint sets seemed to boom in popularity around then. Second, I have noticed perhaps more of a drop-off in condition of pre-1956 (not 1958) cents, and that *does* begin to coincide with lower mintages. For instance, I'm sure I've roll-hunted a couple AU 1957 cents. But yes, in old stashes of cents where BU coins exist, I guess you're right about 1958 being a turning point. Third observation: I think the same kind of thing may be true for other denominations.
PS: I have to admit... I had a big laugh reading your other post and seeing that you were kind enough to rename it for the hijacker! I was going to volunteer to re-start this thread, but you beat me to it!