I think both swisschocolate and Instigate are right for different reasons! It might be useful to look at the tables on page 23 of this: http://www.ets.org/s/gre/pdf/gre_guide.pdf. It shows the percentile score for each scaled score. This lets us know how many test takers scored in which range.
In the ranges of scores 145-160, each scaled score point is between 3 to 5 percentile ranking. This means that every extra point earned means you just scored higher than a lot of people (e.g. if there were 1000 test takers, getting one more point puts you up 30 to 50 rankings higher). However, at the top level, say between 165-170, an extra scaled point is only 1-2 percentile rankings -- scoring higher only means you move up 10 or 20 spots in the overall rank. So, an extra point in the mid-ranges mean a lot more in terms of absolute ranking! Assuming that the pool of applicants to a certain school can be represented by the GRE score distribution (not perfect, especially not for the highest ranking schools), this means that if a school was ranking the profiles by GRE scores, having an extra point in the mid ranges is more likely to move your application upwards on the pile than an extra point at the 165+ level. This makes sense -- if you scored, say 168, you're probably at the top of the pile, so having 169 doesn't really mean you'll move up.
In addition, depending on program, some places treat GRE scores as a "cutoff". The cutoff would be nearer to the middle of the score ranges so having an extra point there might help you pass the cutoff, or come close enough so that they at least look at other factors. If you're 165+, you're well above the cutoff, so having more points won't really make a difference.
So, in these practical respects, I think swisschocolate is right -- an extra 1 or 2 points might make a bigger difference to your outcome if you're in the mid-ranges instead of the high ranges.
However, Instigate is also right in a different sense. In terms of ETS Test Taking Ability, scoring a 168 instead of 166 is much harder than scoring a 158 instead of 156. It's basically "diminishing returns" -- at the tails of the distribution, it's a lot harder to move upwards.
In terms of "making a difference in admissions", I agree with swisschocolate. In terms of "difference between scores", Instigate is correct.
With all that said, admissions are decided on much broader aspects of the application than just GRE scores!!
