I have always been brought up this way. You concede a corner, one of the first things you do is get one person on far post and one person on near post. Usually your two smallest players or your left and right back. For those who know me this is always and forever will be me. However, at professional level you very rarely see players on the post. If there are players on the post, they will almost certainly only have one post covered not both.
It does seem to me that when teams do concede goals from corners, they are often hit straight to where a player would be standing at either the near or far post. That of course makes sense, if the goalkeeper is standing in the middle of the goal, trying to protect as much of the goal as possible the only place attacking players can score is in the corners. That is why I was always taught to have players on the posts, in case the keeper misses it or the shot is in one of the corners.
The number of goals scored from corners varies in between 5% to 10% depending on what study you look at an where they take their measurements. There has been a steady increase in the amount of goals scored from corner kicks in the EPL over the last 10 years. So, there is ether more of an emphasis on scoring from them or there is not as much time devoted to defending them. Maybe it is a combination of both?