If you're keeping noisy gear in the closets, no.
If you're actually going to try to use a closet as a vocal booth (which only actually works in the movies), then absolutely positively yes -- And you're not going to get away with only stacking traps in the corners...
I've had the unfortunate pleasure of retro-fitting a closet or two... Pull all the drywall (ALL of it), fill the voids with rigid fiberglass, cotton duck over that, strips on the studs -- Still sounded like crap (I mean, it's a frigging closet), but at least it wasn't resonating in the baritone range...
AFTER READING AGAIN: Okay, I probably shouldn't assume you're trying to turn closets into vocal booths -- Which would be great. If you're simply trying to trap the closets, that's a stellar thing to do. The more the better as at this point, they're simply resonant chambers. On the positive side, BEING resonant chambers, they can be quite effective as giganto-traps.