A little bit of both. At first I just placed them in capping circles. It kills a guy just over half the time from my experiences. The intel + the chance to kill makes it a lot easier to know what kind of units you are going to need to fend of that capping squad.
Once you get used to placing them, you can start to place them on flanking routes or near building entrenches to see if you are getting surprised by JLI and the likes.
These days it's also a lot easier to place them because you are no longer required to have vision on where you are going to place it. You can order them to place one in the Fog Of War, and have them shift-move to some other stuff. You could not do this at first because you could only place them where you had sight, which made it a big hassle to place them.
But anyway, we are steering too far off topic. The opening post has many great suggestions (bar the flare mines in my opinion

), lets focus on those again!