No, you do not need a cycle computer or a GPS device if you have the updated maps. Especially if you say that you are good at reading maps. I have never really looked at the mileages on the directions on the left of each map section. If you need to use the mileages on the left, you need to constantly calibrate your computer. I use my cycle computer to track the instant speed, the average speed and the total daily mileage. But I never use it for orientation.
Particularly in the hills of eastern Kentucky on the TransAm, I was reminded of the ancient computer game. "You are in a maze of twisty little back roads, all different." Maybe there was a road sign every 10 miles (3-4 turns). Or maybe not. Without the distances on the maps and an odometer, there would have been a lot more "zxzzy"!