I have been taking it this past week for a sinus flareup. Usually my mucus starts in my nose and throat and then goes to my lungs and I get the dreaded cough. Mucinex has helped keep my cough at a minimum and very loose, so I can cough it up easily. I have taken it with much success. I guess if you have a ongoing problem with mucus with your asthma it might help to take it all the time. I usually don't have mucus in my chest unless I am sick or sinus.
I am like you; my asthma doesn't bother me unless I get a cold or sick. The minute my throat feels scratchy, I know it's time to take action. This last time I tried Mucinex, but I didn't start it until after the cough started. But it helped a lot. My cough was very productive and along with my other asthma medicine, this flareup lasted less than a week. I think next time, I'll start taking it the minute I feel the drainage and the scratchy throat. But I recommend it!