Interesting. I had never really thought about it.
When flying a towplane (Scout or Citabria or Super Cub), I almost always slip to the left, mainly because I tend to fly curved approaches in a left-hand circuit. This also gives me a better view of the runway since I'm landing on the right side of a grass runway, with the take-off grid full of gliders on the left.
I also prefer left slips in Cessnas or Pipers, because I get a better view from the left seat,