Not only does it not hurt anything, our DD Series 60 DDEC3 will only attain full horspower and torque if the Cruise Control is engaged.
Makes for interesting driving. Approaching a long steep upgrade, make sure CC is set. The interaction between transmission computer and engine computer keeps it in the right gear and RPM. Without the CC set, the transmission will downshift earlier. With it set, the trans will hold in gear longer, and allow more "droop" before shifting.
Not only is it easier on the coach, it takes a lot of strain out of the drive. In all newer truck design, let the computers do the work. That is what they are there for.
Sometimes we will forget to engage CC, and it isn't until we have dropped way below the torque curve that we notice. Too late then, just move into the Slow vehicle lane, and remember it before the next hill.
We drive the class C with the Mercedes diesel and ZF automatic the same way. Works well, but only the manual for the Detroit specifically states that CC must be engaged for max power.