In the Cascades you're probably talking about yellow-jackets, and they're a big problem from late August to early October. Elevation is important. Yellow-jackets are pretty rare above 5,000 feet. If you want to climb an objective that requires a lot of off-trail travel through dead-fall, I would never attempt something like that after mid-August. You'll be tempting fate, as they often like to make their nests in dead trees and in the ground near that kind of debris. I don't remember the Colorado Rockies having this kind of problem, but it's certainly a yearly issue in the Cascades. August and September have been very warm and dry, and I think that exacerbates the problem too. After the first killing frost, you're usually safe.