I'm a firm believer that anyone should do whatever they feel like doing with their books. Write it down, highlight, use post-its, whatever you feel comfortable with. Everybody has their way of reading, and if annotating books makes it easier for you to understand them then go ahead. There're some people that think books shouldn't be annotated because of the whole "keeping the aesthetic thing" but I think it's cool to put your own part of the experience you had with the book in its pages