I had friends whom I was teaching a waltz ask me why there is such a tilt in the dancers' heads when in ballroom position, particularly the lady's.
I said it was a bit more common in International Standard, and that it does help with balance, keeping nice lines and good tension/connection/feel between the dancers.
I also said I thought it helped with spotting and that it can control dizziness a little bit.
Were these correct answers? Or was I way off?

dheun.First for the man there is no tilt of the head , both eyes are horizontal. The lady. Again there is no tilt of the head. What you can see is a shaping to the ladies left by stretching her right side without collapsing the left, Her right eye is higher than her left. Not achieved by tilting the head which would create a bend in the spine.

My friends used the term "tilting," as I suppose a lot of people would when viewing that kind of posture. Your definition explains the technical aspects of the stretch, but are there reasons for it, specific to the dances themselves or for any benefit in completing certain aspects of a dance?
But now that you have explained it, I can see the difference between tilting your head and making a bend in the spine vs. stretching one side while keeping a nice frame.

The lady is usually lighter than the man. Having her head weight further from the center helps balance the couples center during spins.
As far as spotting and dizziness control your were way off.

"dheun.First for the man there is no tilt of the head , both eyes are horizontal. The lady. Again there is no tilt of the head. What you can see is a shaping to the ladies left by stretching her right side without collapsing the left, Her right eye is higher than her left. Not achieved by tilting the head which would create a bend in the spine."

You are correct that it's not really a tilting, instead its a positioning of the head by the entire body.

However you are wrong to say that the man doesn't do it. He must do basically the same thing as the lady, but to a notably lesser degree. Because it's a lesser degree, many less experienced male dancers aren't yet aware that they will ultimately need to do this.

Many people mistakenly think that the way for a couple to look big is to put their arms out, but this is actually not true. A couple with their arms out and their heads central looks small. The big look is achieved by creating separation between the heads - in essence, by each taking their head closer to their own left hand.

Hi anymouse
Would you say that the head position (for man and lady) should exhibit a smooth continuation of the upper spine?
For the man in normal closed hold, for example. The upper torso is rotated to the right slightly to keep the man and lady's centres towards each other. This gives a slight leftward (and upward, of course) shaping to the upper spine, and the man's head extends this in a natural way (or should). Standing in closed hold with both facing 'forward' would leave the man's head central (the spine being vertical) and, as you say, the picture is smaller.
In line figures like same foot lunge, the man's upper torso is more strongly inclined to the right, and the head follows this line. On the recovery before exiting, the man breaks back to the left (with strong rightward rotation), and the spine is up and out to the left. The man's head again continues this line. Hence the concept of keeping both sides of the neck muscles equally toned. If one side is 'pulling' more than the other, the head is to one side and won't look so good.

This is all helpful information about a topic you don't think much about in general, but people do ask when you are teaching them or watching ballroom on TV. As for the dizziness part, I seem to recall reading a posting on this board that someone found it easier to cope with the V. Waltz by making sure you kept moving along the LOD, but also by tilting the head a little more. We tried it, and maybe it was a mental thing or something, but it did seem to help a little. But it might have looked a little odd. Also, from a practical standpoint, there may not be a lot of logic in that.