I would say this question gets the hated answer, "It depends.. "
If you or have had MAC, 7..% saline is demonstrated to suppress the growth of the bacteria in your lungs. Even if your airway clearance doesn't produce sputum, the saline will be in there keeping the sneaky pockets of mucus from being a breeding ground .
On the other hand, I'm not sure what if any research has been done on whether it is helpful if you have never had MAC.

There is a frequently asked companion question, "Should I continue airway clearance if I don't produce mucus?"
The answer is Yes!
This will prevent mucus pockets and plugs from forming, which are an ideal breeding ground for MAC and other infections.
Are there any circumstances under which you produce mucus?