We've never done the "formal" imprinting that is written up in books.....but we have done a mild version. By that I mean, after the mare has bonded and while we are rubbing the foal down with a towel, we use the towel to dry and handle ears, legs, etc. And then the next day we use our hands and touch ears, legs, and in general just pet on it. We do this every day until the mare and foal are turned out with our "baby group" by the barn.
I have noticed that the foals are friendlier when we do this....and will also add, we have had a bottle baby who we still have as a gelding. And yes, he can be a little crap! LOL.