Well all times of year have their good and bad I guess, and its different in different places.
I decide a couple weeks ago the best time is that first week or so the local schools are out (mid-late June) as the traffic suddenly gets light and the tourists haven't quite come out in droves yet. Come this week, San Francisco will be packed with tourists walking and riding rental bikes along a major part of my route, and the golden gate bridge will also be packed with rental bikes at night and the occasional early tour group in the morning.