Lots of people do this, stutterers or not. But it can be a bad habit, for sure. For us, it's especially fickle if the filler sound itself becomes part of the stutter. That means we're spending even more time outside of that we were meaning to say. But as far as I recall, it can be used as a tactic for easing into the onset of tricky sounds. I just can't remember if this is a legit speech therapeutical technique. Depends on the precise manner it's carried out in, I suppose.