For the first time, absolutely NOT! It's extremely off-putting and disrespectful. It'll make them not want to converse with you again. I'm only comfortable and condone allowing people I have rapport with or know me very well to finish my sentences when I'm having issues because they more likely know what I'm going to say, and it speeds up my conversation with them. All of my close friends have asked me if it was ok, and then I give them 'consent'.