I would say yes, you should push it a little on the days you can. I would say especially on the shorter ones, although some would not agree.
OK: let's start with 6:40 pace.
This is easy because it's an even 400 seconds. You may be faster; I'm actually a little slower now but this was my M pace once....
10% is 40 seconds.
90% is 7:20
80% is 8:00
Now for me at this point I would try to do a medium-long run at 7:20 for sure. In fact, I could do something truly considered 'long ' at that pace if I wanted it to be hard. And like I said, under 7 is a tall order for me for full 'thon.
Now, 5% is :20
95% M is 7:00
So, I believe you should be able to do a fair amount at that pace, at least medium-long. Maybe some runs can be easy, especially early in a cycle and/or when you don't really need or want to go deep (or can't for whatever reason, like hangover from previous days or weeks). But I think you'd agree that 8:00 is real easy for anyone who is racing a half at 6:40, let alone a full.
Like I said, my short answer is yes. Change the 80-90% to 90-95%. Or 10-20% slower to 5-10% slower if that's how it's written or how you think about it. A least a few long runs should be at a solid 90% and several medium show definitely be 95%.
Let me tell you that I believe long runs are the most significant specific training for an event as long as we're looking at, so maybe I put more importance on them than some. It seems to me that these should be hard workouts, or at least half of them should be. I believe Pfitz agrees, although his book isn't in front of me right now. He and I both agree that a bunch of 10km training (quite a bit faster than goal race pace) isn't ideal, so it seems we're coming from the same direction. I just think he should have cut the 10-20% in half to 5-10%.
At least that's one opinion. Good luck!