I keep left on all one way avenues, mainly to avoid the buses. 5th Ave along the park is an exception, and I might switch over to the right side depending on traffic. It's a matter of balancing the problem of left turning traffic every other block vs. playing leapfrog with buses or having to swing around stopped ones with the tails hanging out. I decide based on the number of buses which varies with time of day, or more often with how they're running. 5th Ave buses often clump, so I'll ride right in the gaps, and as I come to a clump switch to the left side. In heavy traffic, where crossing the avenue is harder, I'll follow my basic rule and keep left.