I doesn't matter before the fact. If I know that 1 out of 8 encounters will be by the book, then before the encounters it's better to pick up the book and just use it to know things. It will be apparent to an experienced player within in the first 5-8 encounters how many you've changed. After that you just play the odds.
Why would I treat every encounter as if it could be different just because you occasionally change one? If you don't change close to half(and even then it will probably be worth it to rely on the MM), then I'm not even going to think about it. I'd just shift tactics if something doesn't work the way it's supposed to.