It is okay if you mark in your Bibles. ¬†Underline stuff. ¬†Write references to passages that relate to the one you marked. Draw pictures to illustrate the truth or to help you find related passages quickly. ¬†Your Bible is a legacy to those who will come after you or even think of it as a marked up road map for those who will see look through it some day.
As good as it is to mark your Bibles it is more important to have your Bible make a lasting mark on you. By a mark I mean let the Bible change you, influence your life, mark you.