I check beside me and my mirrors so often, that helmet cam video from me would likely make you motion sick.
Others have said it but it bears repeating; the blocking positions are not a hard and fast rule, but rather a guideline. Each situation must be evaluated separately, then actions taken accordingly. The blocking position is constantly drilled into you as the magic bullet. It isn't.
Firstly, it is described as riding in whichever tire track. You would be better off riding just inside that tire track, as it creates a situation where it is less likely that someone will try to lane share with you. If another driver sees what appears to be a 90% clear lane then that person will likely try to take advantage of it, especially in a downtown heavy traffic situation. If they see two thirds of a lane? Less likely.
Secondly, there are times when the 'classic' blocking position makes less sense than something else. You want to block in the direction that trouble is most likely to come from both in order to be more visible, and to create a deterrent against someone changing lanes into yours. For example if you are on the highway and coming up to an on-ramp then it is more likely that someone would see and try to take advantage of what they think is a space to merge into, than someone to your left is likely to make a change into the slow lane. In this instance it generally makes more sense to block to the right.
When making a left turn on a multi-lane road most people will tell you to block right, so that people don't try to go around you. I generally am in the left blocking position, and so not blocking the lane to the right. I do this because it puts me in an advantageous position to both see, and be seen by, oncoming traffic. The further ahead you can see, the easier it is to react to what's coming. Remember that I said the blocking position (for me) is about a third of the way into the lane, and so you still provide some deterrent against people passing you in your own lane.
On highways with 3+ lanes in each direction, try to avoid the middle lane(s). You can't block and it's far more likely that someone will intrude upon your space. Sure, there are times when you don't have much of a choice, but try to minimize them.
But none of this mitigates against simple situational awareness. Keep your head on a swivel and try to take note of the vehicles around you. If you suddenly don't see a car that you noticed just seconds before, that's time to worry.