I don't see any issues with the techniques proposed.
Naturally, they all have their own advantages and issues.
I have used the Sky Mounti, which is a curved bubble level type of device. It was handy for getting a general idea of the slope of the road while riding. Like anything that relies the relative gravity vector, any sort changes in your speed will affect the indicated slope. For best accuracy, you'll want to stop and get a measurement.
I've got a spare Sky Mounti, in case anyone is interested in trying one out at a low price.
The phone-based inclinometers do fairly well, based on some quick experiments that I've done. Certainly good enough for most of us.
In most cases, I don't tend to worry about the slope of the hills I'm riding on. It can be handy to know the slope of some roads, if only to know how steep they really are and what gearing is suitable. It's also helpful when describing a hill to a friend, when they want to know how one hill compares to another. If, for instance, they know that they can climb up Mossville road, but aren't familiar with Cedar Hills drive, I can tell them that both are steady 14% grades and should be equally difficult.
Cedar Hills Dr. has a better shoulder, though, so I'd recommend that as the best way to get out of the river valley.
Steve in Peoria, where our steepest hills are actually valleys.