It doesn't take much for the conditions to change. We were staying at Yosemite West during the Rim Fire. For the first few days, although it looked like Armageddon when you stood on the south rim and looked north, we had wonderfully clear conditions where we were. After a few days, the wind shifted overnight and we had an uncomfortable but "doable" hike near Bridalveil Creek CG in the morning. We went down to Wawona for lunch and by then, the smoke was so bad that my wife wanted to leave the Park immediately. We stuck it out and, by that evening, it was fine and we had no more problems for the rest of the trip. So, I think the most you can do is see how things are trending and then take your chances.