I guess it would depend on your purpose of estimation.  If used to prove a calculation correct, then spend a little time on it.  Otherwise, in this day and age with the use of technology, spending time on this concept would only slow the student down when they are moving on to higher mathematics.  The estimation process should have been covered in JH.  As far as mental math goes, encourage the student to not depend so much on the calculator as it only slows their progress.  Show shortcuts in calculating.