You could cross just north of Nevada, then cross California north of the Central Valley and head south on the coast to Santa Cruz. In my experience, the heat is much more miserable in the Central Valley.

You'll have to decide what you want on your tour. Do you want to hit the Grand Tetons? Do you like quiet routes between funky little towns (who doesn't?) Camping or indoors as much as possible?

What I would do is look for some waypoints I would want to hit along the way, then search for other tourers that hit those places on their routes on crazyguyonabike.com. Their journals can provide invaluable information. Google Street View can show you what a road is like. There's a good reason bike tourists don't cross the Sierra on Hwy 50, for example.