I just did some tests, while Needle Bow and Wugong Gu are both listed as missile, Steady Aim has absolutely no effect on them. I tested with and without a weapon (to get rid of damage variation) level 10 steady aim, level 1 wugong and needle. With weapon, there was some variation, but damage didn't seem to show any statistically significant change. Without weapon, doing just flat damage, there was absolutely no increase in poison damage with steady aim. I'm not sure if unequiping a bow gets rid of missile property, but the first test showed no noticeable change either. I will test this again once the patch hits (hoping that latent venom is missile).