Personally for me, I read from a few resources (forget where liao) that should use 1/(focal length * 1.6).
Since my lens has IS which will give it around 2 stops. This 2 stops just nice will cancel out the crop factor.
1/(focal length *1.6) *2 is about equal to 1/focal length.
So in the end, I still use 1/focal length. My 2 cents.