The Klingons have come a long way from their first appearance in the original Star Trek back in the 1960s. They might be fierce (if not vicious) but they have neither the down-right nastiness of the Cardassians or the untrustworthiness of the Romulans.
The original Klingons had no prosthetics except for dark makeup and the hairdos… beards and mustaches and bushy eyebrows. Then in the features they added subtle foreheads … and some not so subtle. When the TV show came along, I added the nose to go with the forehead and over-emphasised the bones in the forehead using for research cross-sections of dinosaur vertebrae.
However, a time travel episode of Deep Space Nine once again had ridgeless Klingons astonishing other characters and sparking a terse reaction from Worf (‘We don’t talk about it’) – not to mention puzzling those fans who care about such things. And more recently the makers of Enterprise with their almost unfailing ability to remove anything of interest from the Star Trek universe felt the need to explain the history of the ridges.