Swanny007 - 4:51 am on Jul 1, 2013 (gmt 0)
336 works on some locations on some of my sites and the 300 works better in other spots. Do an A/B test on both sizes for 2 weeks and see which has a higher RPM.
Oddly enough, I did test 336 vs 300 after Google announced they backfill the 336 ad with 300 ads, and the 300 ad performed better (RPM), so I went with the 300 size. Overall though just putting in the 336 is the way to go if it fits with the site's design.