In my opinion, torso rotation is the most important skill set for good stroke technique and following the blade with the eyes helps to develop strong torso rotation. If you look the other way the body isn’t able to rotate fully which in turn means less power.
The purpose of practicing good stroke technique in flatwater is to create muscle memory that you can call on in the rapids when it’s harder to focus on perfecting technique. So, it makes sense that you would follow your blade with your eyes in the sweep and stern draws to practice torso rotation in flatwater, but you do always want to look where you’re going when you’re running a rapid.
Remember that it’s very important to practice your strokes in flatwater to help develop efficient strokes which, in turn, will make your paddling more graceful and powerful.