Even if c-stick is set to smash, you can still use it for aerials. You just need to tap it instead of holding it. However, for most characters, I find this is a good enough reason to set it to attack. For characters with good tilts out of perfect pivot like Little Mac though, c-stick attack is a must.
However, if your character has a good Dash Item Toss Cancelled Item Toss like Mega Man, or makes good use of smash attacks out of perfect pivot like Fox, you may want to keep it on smash.
Alternatively, I find that Mewtwo, with his good special move options out of perfect pivot and a teleporting up-b, does best with C-stick set to special.
Ultimately, it depends on the character, and which options you value using most. You should make your most important options as ergonomic as possible.