Right, this is an After Effects question I presume.
I suspect you're talking about the futuristic front door number tag tutorial on the videocopilot series.
In that case he wants to track an existing plate on the wall. To track that for perspective corner pin what better than the four corners. If I remember right he uses the screw headsused to mount the thing on the wall. but why? I think Andrew does mention it in passing but a good track point in that kind of example is a small shape that looks the same and to the same scale throughout the clip. i.e. the screw head is the same shap and size throughout the small clip he uses.
Also, the track point needs to stay in shot throughout the clip as well. Let's face it, you can't track it if it's off the screen can you?
So then, anything that stays the same through the clip but changes position is a good candidate. The track point works best if it has good contrast against the area around it so it stands out and can easily be 'found' frame by frame.
I hope this makes sense. Ask if you need more.