Thank you---I'll keep this in mind when I ride on the avenues again. I wasn't aware of this practice. In the case of the 2nd avenue, however, there is this dangerous intersection at 59th street, with cars coming down (ususally speeding) from the Queensborough Bridge on the left side, so the right side is definitely better there. Another factor: the on-going subway construction is on the left side of 2nd Avenue.
Yes, that's a tough area. You can either cross to the right in advance when traffic permits, or use the lights to advantage to work your way through.
My keep left advice is general, and obviously needs to be adjusted according to specific conditions. One nice thing about NYC riding is that you'll never have brake shoes dry out and harden.