For me, approximation problems were some of the hardest and slowest problems on the test.  Even though in number sense you are allowed a 5% window, they can still be very difficult at times. 

In general, if the answer is going to be "large" then there is a greater leniency in what you can do.  If the answer is going to be "small" the 5% window will not be very much so you need to be more accurate.