I don't remember anything canon, saying either way, so this is just conjecture on my part. I think the Klingon Neutral Zone, at least the side shared with the Feds, may have been treated more as a border by the time of TNG, than a DMZ, because the Enterprise crossed into Klingon space pretty frequently. There may have been nominal forces still stationed, just in case, but I don't see two allies having a heavily militarized border.
Now I am sure the Romulan-Klingon Neutral Zone was probably highly fortified on each side. For obvious reasons.