Hi,
The idea of "where" to look isn't really the most important, I feel; rather, the emphasis should be on "how" you take in the opponent(s). In this, there is little better advice than Sukerin's first comment on Zanshin, and this (no matter what particular terminology you or your system use) should be achievable with any technique.
The differing schools and arts will have various answers to the exact place your eyes whould rest, but all will have this concept at the heart of their reasoning. The idea is to be aware enough that you are able to see an attack coming, and also be in a position where you become aware of any openings, giving you attack possibilities and counter-attack options. This is predominantly achieved through a sort of "diffused focus", where your eyes settle on one or another point (the shoulders, the centre of the chest, the movement of the hips etc), but you maintain your peripheral vision to take in the entire body. This is not only a useful thing for the above reasons, but also because under the effects of adrenaline, your vision will tunnel, and the only way to keep your peripheral vision in this situation is to have trained (ideally under adrenalised circumstances) to be aware of, and maintain it.
But to give some examples of those methods taught, the Koto Ryu teaches it's practitioners to look between the opponents' eyebrows (this is designed to give the false impression of eye contact, while allowing you to keep the opponent in your vision); Yagyu Shinkage Ryu teaches to focus your attention (note: not your vision!) on three various points: between the opponents hands when they are holding a sword ("The Two Stars": to allow you to see the direction of their cut), between the wide part between the elbows ("The Peak and Valley": to see when an attack is coming), and the shoulders ("The Distant Mountains": when the opponent has engaged); the Go Rin no Sho (and by extension, the Hyoho Niten Ichi Ryu) actually teaches that you should never have a particular focus, as that is another limitation, but instead to maximise the breadth and range of your vision, while making the point that the observing and percieving eyes are different things; others teach to watch to the right shoulder (again, based in Japanese swordsmanship), and so on.