Let's all admit it: We do it!!
Personally, I work in LTC, and I know what my residents take and when like I know the "back of my hand". I know the residents,and they know me. I sometimes pre-pour meds, and have never had a problem with it yet. Our pharmacist, however, doesn't like it, and has said that if we must do that, to keep it to a max of three or four residents' meds. I have found that it is often LESS safe to take my med cart with me, because I know that I won't be able to keep an eye on it.
All told, we need to keep in mind that "safe and prudent" practice. I think it depends on the situation and the policies of the particular facility that you work in.