It depends where you're going. Some DOT websites have decent maps. Some don't.

I carry small paper cards and put my own maps on there. They are a combination of RideWithGPS maps with elevation profiles and Google Maps that I print out. Basically they're a quick reference that can answer any basic questions I have. My routes are all planned in advance, which may not be your cup of tea.

I could do an imperfect reroute based on those maps if **** hit the fan. They're good to have, but obviously Google Maps on my phone would be more detailed.