Thanks irc - very useful info, good to know prevailing winds aren't really an issue, I didn't think they would be.
Washington DC- SF - LA looks like a good bet (though what is cycling in LA like?!)
However, it's beginning to look more likely I will need to start in Boston and do a more northerly route. Which would mean having to get a car or public transport to go from say Seattle to LA.
What were the other two crossings you did?
Vancouver to Boston. and San Diego to Florida.
The ACA Pacific Coast route goes past LA so getting to LA wouldn't be a problem. I've never found any ACA route to be a problem for a cyclist used to UK traffic. Bar a 400 yard stretch of Route 1 south of San Francisco where I pushed up a hill with fast traffic, bends, and no shoulder.
Drawback of a straight line from Boston to Seatle area is that the Great Plains are a bit wider and emptier further north. Crossing the plains on the ACA route through Kansas is a nice experience. You are needing to get south to get below the Great Lakes anyway. Once you get far enough below Chicago to avoid the big city traffic effect the distance to any point on the Californian coast from San Francisco north is pretty similar.
For a better idea of my DC - SF route and a look at the scenery my crazyguy blog is at. http://www.crazyguyonabike.com/doc/ridingwest
For me going north and having a week on grassy plains rather than a week through Utah would be a huge mistake as far as scenery goes,
For another Boston - San Francisco route try Mike Noonan's journals. He's done SF - Boston 3 times by non ACA routes, though he uses mostly the ACA Western Express from SF until across Nevada.