I don’t see any better outlet for this type of recommendation - I would love to contribute more to CI as my team and I have been using it recently on several projects.
Anyway, instead of sending a default value of false if a parameter is not set, I would recommend that it be a third option in the function.
function post($index = '', $xss_clean = FALSE, $default = FALSE)
if ( ! isset($_POST[$index]))
Instead of returning false, I could assign a default value (which, by if not provided is false). I frequently need to assign something else, like a string or a non-zero integer.
I would recommend this for the get/post functions, if not the cookie/session functions as well.