I agree with Tim: it could be a problem in performance, but as long as it's limited to practice it should be fine.
In fact, there's a great historical example of it being troublesome in performance: the Canadian pianist Glenn Gould was (in?)famous for humming while playing, something only made more uncomfortable by the fact that it was often off-pitch!
Now, you may think "if this guy can do it, why not me?" Well, perhaps so. But keep in mind that Gould was a once-in-a-generation performer and musical thinker, and so the quality and innovations of his performances help us overlook the humming. It's also possible that recording technology at the start of his career helped him become famous before his humming became all that noticeable, but that's just conjecture.
In any event, singing/humming as a practice method can be incredibly helpful as a means of clarifying your interpretation of a melodic line. My personal suggestion would be to do it away from the instrument, and perhaps only occasionally while playing. Because in performance, nerves often result in us doing things we don't intend to do, or doing things that comfort us. Thus you may find yourself getting nervous and humming along without even intending to, which could be problematic.