In practice, one can and should "target" the kick. In a real situation, I think the only rule of thumb is to kick no higher than groin. With the toes of the shoe, especially dress shoes (minus high heels), anything you hit solidly will at minimum get serious scrapes if not painful bruises. Yet, I would not count on the kick(s) alone to stop anyone. Though these can in some cases, many more times they don't because of the adrenal rush. I witnessed a friend (a uechi dan student) kicked an opponent THREE straight times in the groin. While the guy felt it, it didn't drop him like "magic" as we are sometimes led to believe...
Rather than thinking about (and perhaps being overly reliant) on any one specific technique, people should practice techniques as part of a continous attack in which the distance between the defender and the attacker closes and the techniques employed changes accordingly, e.g. you go from a low kick(s) to punches, to knees and elbows and, maybe, even to grappling. Basically, you try to give as many hits as you can. going in on the opponent in a totally committed, "offensive" mode. The idea is to not rely on any one specific hit (or technique) but on the cumulative effect of numerous hits to overcome the opponent's adrenalized state.
While targeting is important, I think sometimes it's possible that too much emphasis on the this leads to a programming of "single technique/attack) in the body's memory. I think it is more preferable to program the body to launch continuous array of techniques. In the latter, the kick is launched at any (low target) available -- shin, thigh, groin, knee, hips, whatever -- to be followed by another kick, punches, etc. There is not a lot of time in a real situation. You take whatever you can get in, even if it is less than the ideal target.
The exception to this will be when the opponent has an edged weapon. (Of course, the first and most peferable option here is to RUN!!!) If you have to, then here you want to target a bit more and hold back from rushing in (to a slashing/cutting/stabbing counterattack). But, once the knife is "passed", you grab or hamper the knife hand and go into "coninuous" mode with your other hands, knees. At this point and range, even if the opponent were to get his knife hand free, you must keep hitting with everything you got. Stepping back is to invite further injury and, perhaps, death.
I know the kyusho folks will advocate "targeting". It don't necessarily have to be "either/or". Work on your targeting in practice but also your "continuous" attack. On the street, just don't stop until the person is down.