In reply to Crag_chimp_uno:
The national park service has some stats:
http://www.nps.gov/yose/planyourvisit/climate.htm
My money would tend to be on October, but I've been wrong about this sort of thing so often it's beyond a joke.
Spring is magic for the waterfalls. An advantage to autumn is that if it is hot in the valley, you can always buzz round to Tuolumne. Spring is a bit more iffy for Tuolumne as the Tioga road can be closed until fairly late if there has been a lot of snow in the winter.
SuperTopo might be a better place to ask this.