[PaleMelanesian]

Some autos are quick to release the transmission and go into a semi-neutral mode when you let off the gas - GM models in particular. Others, like Hondas, are very strong on holding it in gear, giving you heavy engine braking whenever you let off the gas. Clearly, in the 2nd case it's more advantageous to shift to N, while in the first case you really don't need to, since it's already doing it for you.

I would hold speed at 50-55 mph. Cruise control is good on flat ground, but as hills come in, you'll do better with manual control. CC is a reactionary device, whereas you can anticipate. CC will overreact and floor the gas, causing a downshift. In the same locations, you can hold it in top gear yourself.