What if they're bald and have a flabby neck? Eye level?
Ask the person to learn forward, towards you and the camera (I don't recommend using a 50mm or wider lens for this!). This reduces the size of a neck or double chin. Most people tend to shy away from the camera and automatically lean slightly backwards.
Beyond the man-woman thing, though, what about view camera movement? When I take pictures of a building, if I point the camera up, i get converging lines which looks 'wrong', so instead of pointing the camera up, I use front rise, which gives an effect as if the camera was higher than it really is...I THINK the same effect could be achieved by raising the camera on a ladder or something. In portraiture, say for a full-length shot, should one put the camera at a certain height and use rise to achieve the perspective change, or raise the camera itself? Or is the effect identical?
The following photos were taken with the camera about 12" above ground level using a heap of rise and tilt... The subjects are looking down
Cleared the bowel problem, working on the consonants...