No matter what kind of track it is or in what game or real life scenario you are in, the fastest line is the smoothest, widest (meaning the arc of the turn), shortest line around the corner. We all know this. I am obviously not telling you something new. Any good driver will look for and use this line.
In my opinion, the physical aspects of the race, the buildings, light poles, benches, trees etc. construct the boundaries of the course. The checkpoints represent the co-driver or navigator in directing you through the course. Again, in my opinion, if the checkpoints become an obstacle or liability in the form of being difficult to hit or easy to miss then something has been done wrong. There is nothing wrong with wanting to keep the race lines on the road but best practice would be to enforce this desire by using well placed props to remove the option. Doing so with the checkpoint placement is a legitimate option but it leaves the possibility of a driver missing said checkpoint on the inside. I can tell you from experience nothing pisses a driver off more than being in a tight race and trying to squeeze every inch out of the track, taking a risky line to try to get an edge and then missing the checkpoint. One simple and visible prop in the opening would tell the drivers not to go there.
Also don't get me wrong, on many roads you can put the edge of the corona at the edge of the curb and it will still feel like it is in the apex of the turn. However, putting the checkpoint in the middle of an intersection does more than remove the shortcut, it also removes the more subtle cues that a good checkpoint placement can provide. A checkpoint placed in an apex is also going to be off to one side of the road and partially into the corner which all on its own gives a lot of information to the driver.