Adding more to my last post:
In my opinion, one reason, really, why folks opt to block with the jo, in techniques like makiotoshi and makiuchiotoshi is that it is a lot "easier" to do it that way - in the same way that it is easier to block in Yokomenuchi Irimi Nage (omote). It's much harder to blend with the attack and/or to penetrate through its Yin aspects to its source.
If it might help, here is a video where you can see makiuchiotoshi and junteuchiotoshi being used against a bokken. In the first and third pairing, you can see (what I consider) the correct timing used (involving no blocks). In the second pairing, you can see my student (2nd kyu at the time - still learning) utilizing the incorrect timing and me being forced to not really penetrate in the cut past his jo (unprescribed) block - thus not stressing his hands/arms to the likely point of failure.