Depends

Here in NC most of the backroads don't have shoulders, in many cases you are lucky if the fog stripe is still there and not eroded away. Many of the older US highways and some of the newer upgraded roads have decent shoulders, I will ride them, especially if the traffic speeds are higher. One example is the road I take to town, it is a divided four lane posted 55mph (average speed is well above that) the shoulders are usually pretty clean, by being on them I am out of the speeding traffic and still have room to move around if necessary, unlike the narrower two lanes with no shoulder, on those I have to center up on the lane and hold on to my space. In Nebraska or Colorado? along the I-76/80 corridor they allow bicycles, but you are required to use the shoulder, which I found interesting because there are lighter traveled US highways paralleling the interstate through those areas.

Aaron