Where do you look when you aim?

I always seem to have to aim lower than what I want to hit.  Probably wrong but kinda sorta works for me.

The topic of "aiming" is somewhat tricky and it can mean different things to different people. If you are someone with any experience with firearms, or even archery (depending on style & equipment), it will bring to mind a somewhat more deliberate, precise process. When we use sighting systems, more often than not, there are a string of reference points in or sightline that we try to line up and keep steady through the release of the projectile. 

No such equipment or system is in place when throwing a dart. Using a flight blade as a sight post isn't going to work. Looking over the tip? same thing. Some folks may do this and swear by it, but it's more likely that it's the sum of other things in their form and mechanics that are producing the consistent results they are attributing to "aiming" that way. 

Some folks are able to use the small aperture that may appear between their thumb & forefinger as a sighting lane. Being able to do so is entirely dependent on one's grip- if your thumb is more directly under the barrel, than window probably isn't visible, whereas if it's more to the side (think "pencil grip" as many are initially taught to hold a dart that way), then it may be, but that will also depend on where you hold the dart prior to cocking. If you are someone who uses their chin or the corner of your mouth or the tip of your nose, chances are that window - even if it's present - never actually intersects with your line of sight (LOS), or it happens too quickly (possibly even twice) to be of any real utility. 

My guess is that most shooters are purely instinctive. They target by focusing on the point they intend to hit and spend a lot of time and energy trying to refine their throw mechanics and overall form to make that process as automatic as possible. (That's probably why we see so many repetitive answers in groups and forums saying "practice!" and "consistency!" Those are useful things, but just repeating them isn't very insightful because HOW we practice different things informs that which we are making consistent and that repetition could easily lead down a path of doing things that don't work too well and when those become habit, it can be a very dark, deep and confusing rabbit tunnel network to back out of). 

Sorry to prattle on, but it's a question with a wide breadth and there probably isn't any one answer that can or will work for everybody. It can also cause people to dismantle their whole throw all the way to the ground trying to figure out :/

Regardless of what may work for you, the keys are always going to be comfort, repeatability, results-based and heavily rooted in your body's natural alignment to a point of aim. 

It may be useful to do a little research on instinctive shooting in archery, where the archers keep both eyes open when addressing their target(s). That would be where I'd start.

(PS- consistently and forcefully shutting one eye and squinting as you move a dart back n forth in your sightline can legit lead to some vision problems and/or actual headaches. The small eye muscles responsible for focusing will fall out of sync with the other eye and that can be an issue. I've had 20/15 vision my entire life and now, after a few years of trial & error grappling with aiming styles, I have lost some of that acuity in my non-dominant eye :/ So be careful!)