Buco, You were wise to ask this question.
IMHO: Applying ballast is largely an aesthetic decision. I am not aware of its presence having much - if any - effect on suppressing noise.
The one bit of enthusiastic advice I would offer would concern how often you think you might be changing the track plan in future. If you are absolutely certain you will never change it, perhaps, you could go ahead and add ballast. However, if you think you might want a change in the track-plan someday, I would definitely not add ballast. You'd be asking for an awful mess to have to deal with in sweeping and scooping up all those miniature particles of whatever.
And if you do decide to add ballast, I would wait a long, long time before drip-gluing it down into permanent place. Once glued-down, you will never be able to remove it without destroying the track - without exception.
Forewarned is forearmed (!)
Just my opinion.
FrankM
P.S. The ballast on my layout, which you can see a section of here, is not glued down. Through time, it has tamped-down a bit, here and there, from the movement of the trains, but I did not consider gluing it into place due to my occasional urges to change the layout, even though I have not changed this particular track area in 10+ years.