Another stammerer over here
My job is heavily telephone based (IT Service Desk) and I find it incredibly rude if I block having answered the phone and somebody decides they can't wait for me to finish and carries on talking anyway. The worst thing is, I see most of these people on a regular basis and having worked there for 18 months, everybody knows that I stammer.
Having people repeating "hello" at you just makes it 10 times worse. If I'm lucky enough to get a word out or make a sound (very bad habit, but I use the word 'erm' at the start of sentences to allow me to bounce off it), some people will then butt in and start talking over me because they know I'm there.
Of course, this is only a small minority of people. Most are very patient and wait for me to finish.
On the other hand, there are people who'll finish sentences off without even thinking about it, they'll do it to everybody. They're not singling out people who stammer... even I've done it when speaking to fluent people. It's quite easy to tell who's doing it out of habit and who's doing it to 'help you out' though
I don't think any of my close friends have actually ever finished my sentences off for me, so because of that, the way I'd like to be treated is for people to be patient and let me finish. It may make the stammerer worse if they think they have to force the words out and beat the listener to the end of the sentence.