YES! YES! YES!

Mental math is a very very effective tool, i will say rather essential tool for people who wants to get a perfect score in GRE Math section.
With the advent of the calculator on the GRE, some would say the days of knowing how to do mental math are obsolete. While you could rely on the calculator for the entire test, doing so may actually slow you down. Indeed a facility mental math will save you a lot of time, and may help you avoid any careless slip of the fingers.

Discontinuity during solving

Momentarily stepping out of the problem, so to speak, to attend to the calculator can interrupt the flow. You may forget exactly what the number pertained to. Was it the answer? Or was it one of many calculations you need to do to arrive at the answer? If it is the latter, then you may forget your place and have to start over.
Especially during the exam using a calculator can be very counter productive sometimes.

Speed

By constantly relying on the calculator you can get a case of over dependency on calculator. Even easy computations has you running for the on-screen calculator. While you may be fast with your fingers, being able to calculations like 14 x 5 in your head will be a lot of faster. And with so many such calculations, those few seconds per calculation adds up to quite a lot over the course of the test.

Slip of "Finger"

Sure, we are all prone to mental math mistakes. But we are also prone to entering in the wrong numbers, especially under timed conditions. If it is a simple calculation, sharpen your calculator brain—don’t let 17 x 3 = 42, because of a simple slip of your finger.

The endless powers of approximation

Some questions may seem daunting, but with just a little mental approximation you can quickly get the answer. What is 15% of 50,200? Well, it’s a little bit greater than 7,500. If only one answer is close to 75,000 then you save yourself the time of having to enter the number into the calculator.

You may balk thinking that you’d prefer the accuracy, but remember working out your mental math muscle before the test will help you become far more adept at doing quick calculations in your head.

Takeaway

Using a calculator makes sense when you are dealing with big numbers, especially in the case of data input. But if you are dealing with smaller sums (say 20 x 12), then relying on math can save you a lot of time. And you won’t have to worry about any slippery fingers.