Everyone's opinions will have different levels of applicability to your sitch, of course, but I say no, don't stick to the schedule if they are interested in tangents, not until high school. They'll retain more if they study what they're interested in, and those interest-led units will work just as well -- better -- as "pegs" for them to hang a more cohesive study on when they need to do test prep and comprehensive overviews as teens. Plus it teaches them to learn, to study. During the elementary and middle school years, when no outside entities care, I take the stance that we can't be "behind" if they have filled up eight hours a day with learning.
My oldest is thirteen now and has jumped around a lot, not finishing any four-year cycle, and I've decided that I am going to go slowly but surely in chronological order from here on out, one unit at a time. I call it "humanities salon" because I integrate logic, discussion, writing, as well as biography, history, primary sources, and the literature, art and music of the period. I promised myself I wouldn't try to get to any particular date by any particular time. We just go in order, spending as much time as we need in each period. This year we finished prehistory and the ancient middle east and now are in the ancient far east.
He's still interested in other things (Japan, WW2, dystopias, mysteries) more than what we're doing for humanities salon (the Chinese epic "Monkey", evaluating primary sources like Chinese oracle bones) but now I am taking a new position. My stance now is this: he has free time and I support him using it to pursue his own interests, but on my time with him we are going on an orderly journey through the ages in our humanities salon. I have no guilt. Since I did help him follow his tangents when he was younger, he is very good at, and in the habit of, pursuing knowledge he wants to pursue, and lord knows I don't mind buying him books!