A classic. Can be used in almost all circumstances and market conditions.
When to use it: Pretty much anytime.
Why it's smart-sounding: It implies wise, prudent caution, but also a sunny outlook, which most people like. (Nobody likes a bear, especially in a bull market). It sounds more reasonable than saying, for example, "the stock is a screaming buy and will go straight up from here." It protects the speaker against all possible outcomes. If the market drops, the speaker can say "As you know, I was cautious..." If the market goes up, the speaker can say, "As you know, I was optimistic."
Why it's meaningless: It's too general to mean anything. It can accurately describe any market outcome in history, merely by adjusting the unspecified time frame. (If you were "cautiously optimistic" in 1929, you were "cautious," which was good, and you were also optimistic, which was also good. Eventually, the market recovered!)