Should I follow Google Adsense recommendation?

Recently I received a Google message saying that I should replace my 336x280 ad with a 300x250 one. Is Google Adsense recommendation any good? Should I follow their recommendation? The message is sort of like this:

"Replace your 336x280 ad units with 300x250 units
20%-30% predicted increase in earnings.
Ad unit "blah blah blah" is size 336x280. Our tests show you're likely to earn more if you replace it with a 300x250 ad unit. Learn more."

So I tried to search on the Internet for some advice. Here is what Google wrote in one of their help files:

"We have strong confidence in our simulations; but we watch implementations closely to determine whether there's skew in results, and whether we need to alter thresholds. We don't believe there are situations in which one of our recommendations will result in a loss of revenue, but if you have an example case, we're interested in hearing about it."

They sound pretty confident in what they are doing. Alrighty! let's do it! If this change is significant, I will report back in a few months.