I reckon reading the spurious information in textbooks provides some depth to your knowledge and only aids understanding. Also, often in exams the questions will draw on common examples, which may sometimes be present in your textbook. If you want to score near the very high end in exams, it is best to cover all information, even that which may initially seem irrelevant.
For both methods books, there is very little present that won't be helpful. Jacaranda physics is quite good on staying on topic although there are some things that may not be useful. Heinemann chemistry is a good book and all the extra stuff will generally help more than harm. For biology in general as a subject it is good to know a lot of examples.
Of course, each book will have little sections where they go off into an area that won't be examined, but it's best to read through this initially but leave it out when making your notes. That way you don't spend too much time on it but you still have a general idea in case you can use that information in the exam somewhere.