It depends what you mean by "portrait": if you mean it in the sense of a head-and-shoulders (more or less) picture of a person then yes, a focal length of 75mm to 135mm in 35mm equivalent is considered the most flattering - though some fashion (and "glamour"??) photographers will use much longer lenses than that. I like the shorter end of that - 85mm, which is keeping with more modern convention, I believe. Though I have a nice little 100mm 2,8 which works well too.

For portraits - in the sense of a depiction of a person - that include the environment you can use much wider lenses without making the person look too scary. It's all a matter of how much of the frame they're filling.