If it's a bright light, it definitely needs to be angled down somewhat, but how much really depends on the brightness + beam-spread.
You've got a few potentially competing/conflicting safety interests:
- lighting the road/path to see potholes, debris, branches, ...
- not blinding those in front of you (pedestrians/cyclists/drivers, perhaps even animals who'll stop like a deer in headlights)
- making yourself visible (though I use a second visibility light for this)
My method of dealing with this was to get someone to stand on the street while I rode toward them, changing my headlight angle each time, angling it down until they didn't find it painfully blinding. It's not very scientific, but is at least very specific to my particular lights.
Depending on where/how you're riding, you might want to get them to sit in a small car (if commuting, as that's likely the lowest eye-height you need to worry about), stand on the road/path, or get up on a bike (depending on the likely users of your road/trail/path/...).
This should work reasonably well if your lights have a hard cut-off around the edge of the beam, and you'll hopefully end up with the road in front of you lit up (at a useful range). If it's too far, you can still move it further down (duh) but if it's basically at your front wheel you might need to find a narrower-beam light with a harder cut-off.
When I'm commuting, I tend to point it down a little lower (aimed about 3-4m in front of my bike, which is fine for my usual commute speeds with my particular light), but if I'm riding faster, on lower-traffic roads/paths, I'll have it pointed a bit higher (less people to annoy, and likely moving faster so want to see further). Mine happens to be very easy to adjust up/down, which makes this convenient.
Obviously this only applies to lights that are specifically intended to light up the road (not to provide visibility), though I often pass riders who have the former but think it's the latter, pointed straight ahead. I think part of the blame for this lies with light reviews/recommendations, which compare both types side-by-side. I regularly see articles/blog posts along the lines of "2014 bike light round-up review", which include 50–100 lumen front visibility lights alongside 200–1000 lumen headlights, failing to point out that the former are for others to see you, while the latter are for you to see the road & others.