Boy, timing is everything, isn't it? The calculations will depend on your average miles per day/week. Generally speaking, except for the occasional storm, the Sierras are dry through Oct. You may hit a storm or two, but they rarely close the passes for long. And the further south you get, the longer the season. If you can time it so you hit Reno about the end of Sept/start of Oct., you'd be able to hit the deserts without too much misery--although even then conditions can be a bit toasty.
I'm dealing with similar calculation issues for a Continental Divide ride I'm planning for 2012. We need to be done with the highest passes of the Colorado Rockies as late in the year as we can manage. I want to be in Pagosa Springs about the end of the first week of Oct. We don't want to head into the Arizona desert until Nov.
Best of luck with your planning and report back on the ride!