Re: to look at opponent's eyes or not?
Yes, I agree, you must look in a way that you see the whole body - so to speak. This, generally, puts my own eyes in the upper chest area (if we are facing each other). Other body to body relationships have the eyes placed elsewhere. In a front to front positioning - if I go higher I tend to lose the feet (which is a big deal when your practice includes facing kicks but not so much when you don't). And, of course, one's gaze must be flexible enough to move as needed but without losing sight of the whole for the part (e.g. it is generally not a good thing, in my opinion, to stare at the oncoming weapon - such as a sword coming for your head - though it is quite common to see in such cases).
The key is to have a penetrating gaze (not a wandering or catching-up kind of gaze) that does not become fettered by that which it is resting upon.
Still, generally, I tend not to look at the eyes of the opponent because the eyes, like the hands for example, can lie. When a person is set to rest his/her gaze within my own (which is something I check for and can always tell by what they do not see (coming from the bottom), I will always use my own gaze to hide my true intentions - assuming gaps are not closed immediately (which is often the case - and which then pretty much negates all practicality of this question except for the point of letting your gaze penetrate without becoming fettered by that which it is resting upon.)