I'm assuming you are referring to the OG
If so, I would say - yes, you should generally be able to do all the questions in the book. The towards the end tend to be slightly on the more difficult side - though it's not always necessarily the case.
But why bother skipping these questions in practice? Work through them for your own learning and so you can feel more confident going into the exam. If your goal is close to 700, then you really should put your best effort. Do all the practice questions and review your mistakes. Shoot for over 700 and reach your 680-700 goal.
Solutions for select PS questions towards the end (#209-#230) are available here: http://www.gmatpill.com/official-guide- ... id=ogps230
That's correct, my mistake, it's for OG13.
Thanks for the feedback, also a 700+ break means a biased study on the verbal. Also, thanks for the link.