On highlighting words in a book there are two schools of thought yes and no.
Some people say yes you should because it gives you the main points and others say no you shouldn't because you forget to take the information out of the book with you and it can act like stumbling blocks on the next activation layer drawing your eyes to the information you've already gathered.
If you prefer to highlight I would suggest you only do it on your last activation session. It is much better if you create a mind map and add more details to it on each activation layer. That mind map you create will make much more sense in the long run than having to rescan the book for all your highlighted words in the hope that they will help you recall what you have read.
If you have been highlighting in the past and it is for you a sound learning aid then I think you can safely continue to use it. The Photoreading system is for enhancing your reading/learning skills and is intended to add to the skills that you are already successfully using.
Alex