I was listening to the commentary on an episode of Star Trek Deep Space 9, and one of the writers mentioned that Bajorans' facial ridges are more-or-less the same on all Bajora, "unlike the Klingons."
I did a quick survey of several TNG and DS9 episodes, and found that the Klingon cranial ridges were in fact unique. Is there any canonical reference that discusses the uniqueness of Klingon cranial ridges?
There is a lot of Klingon ridge analysis over here, and they seem to come to the conclusion that Klingons appear to have relatively unique ridges, although there is some similarity between related Klingons. Far from definitive or canonical, however.
This doesn't really pertain to Star Trek in particular, but pretty much any feature of the body can be used for biometrics, because no two individuals share identical organs or biometrics in any part of their body if you look carefully enough. It all boils down to relative factors like accessibility, permanence, (social) acceptability, technology, measurability/collectibility, and circumvention.
Whether blood vessel patterns, ear geometry, gait, facial landmarks, body odor, keystroke dynamics, skin texture analysis, or other biometric identifiers are used really just depends on the desired application. So, even though from a human's perspective, Bajoran nasal ridges are all pretty similar, they would probably still be capable of being used for biometric identification.
Whether Klingon cranial ridges can be used like fingerprints also depends on what you're planning on doing with them. If you're trying to dust a crime scene for cranial ridges, then like Jeff humorously points out, it probably won't work. (You'd probably also be hard pressed to get Worf to stamp his forehead on an inkpad.)
It's also worth noting that human fingerprints are significantly influenced by environmental factors, so twins or genetic clones won't have the same fingerprint. This may not be true with Klingon cranial ridges.
Lastly, if you compare Alex Rozhenko's cranial ridges from when he was a child and when he was a young adult, they look significantly different. This is very different from human fingerprints, which are formed in utero and remain the same for the most part.
Around 1993 I read a magazine containing information on Borg, Romulans and Klingons (sorry I can't recall the magazine's title, it's been that long,) but it was very explicit that Klingon ridges are as unique as fingerprints.
