Like others said already, Google is okay for general planning. When it comes to the nitty-gritty, though, it just doesn't cut it. The information is presented in a way that is not particularly helpful to the bike tourist. I do love using street view to get a lay of the land.

When planning a tour I like to use a detailed state atlas. My favorite is the Benchmark Maps Road & Recreation Atlas. The detail is good -- everything down to major forest roads is presented accurately. Also included is the location of campgrounds, both public and private. It also displays which lands are private vs. public, which is helpful for deciding when to camp in campgrounds or not. I use this atlas to plot my specific route. For road tours, the data on these maps is perfectly sufficient. I will often bring along a folding paper map that I annotate by hand with data, like where I will camp each night, where supplies are likely, etc. If my tour takes me through public forestland, I will often consult National Forest Maps. I bring this map with me on the tour. Forest roads are very confusing if you don't know exactly where you're headed!

I also own a computerized topographical map program for the state of Washington. I use this to get accurate elevation data. It is also useful for tours on tiny backroads, as the USGS maps are even more detailed than the Forest Service maps.

I stay away from electronic maps on the road: one more thing to break or run out of batteries!