Your assumption is right - wideangles (< 35mm) will cause distortion making near objects seems bigger (eg nose bigger than normal).
The most flattering focal lengths are between 60-130mm, anything more than that the telephoto perspective flattening effect tends to make "pancake" faces.
But, YSLee is partly right although he didn't elaborate - depending on what you want to portray any lense from super-wide to super-tele can be used.
But for your digicam, the optimal focal length for portraits is between 60-96mm.