Good on you for taking the time to ask. I personally prefer one to just act normal and let me finish. Usually at work, people do. There have been times when I'm ordering something or at the pharmacy, and someone finished or knew what I was trying to say and tried to complete the sentence for me. If I know they are coming from a place of thinking they are genuinely helping me, then I don't get offended. Sometimes, I wish they would do it. But there are times when you can tell they do it out of frustration, and then I tend to shut down and get pretty angry. But I wouldn't recommend trying it just in case. In general conversation, I would just let them speak and act as if everything is normal. One of the worst things for me is that once someone acknowledges my stutter, I can't get out of my head on whether or not they will laugh, giggle, mock me, or even baby me.