A number of people believe that theory.
The reality is that nobody knows, but there is a good chance that GMAC makes it harder to guess questions right, and has a rule (which in my mind makes sense - you want the abilities to decide the score, not a chance). I would probably see it closer to 10 than 20.
Not sure if this is the answer you were looking for, but I would say there i a good chance this is so. Plan to answer every question and spend some time on it. You will get the best/highest score if you don't have to guess any questions - that's the only way to guarantee it.
P.S. If you try to take GMATPrep - it does not have this behavior built into it, but it is missing other features as well, so don't base your GMAT understanding on the gmatprep software.
Founder of GMAT Club
Just starting out with GMAT? Start here... | Want to know your GMAT Score? Try GMAT Score Estimator
Need GMAT Book Recommendations? Best GMAT Books
Co-author of the GMAT Club tests
Have a blog? Feature it on GMAT Club!