Februs:
I would say that it's not unreasonable to assume that they are ALL made up or at least to assume that when reading them. They were clearly published to titillate or to express someone's fantasy through the medium of a letter. I think in the majority of cases that's all too clear to see, a few however are written in a less sensational manner and may appear real but I would suggest that the vast majority of those are also purely works of fiction. I have also had dialogue with more than one person who had letters published in the mags who confided that everything they wrote was made up.
It is really hard to say if "letters to the editor" are real or not. I would agree that most of them are fiction, or at least embellished, but some are probably real. Those with the least description are most likely the true ones.
There once was a weekly mainstream newspaper that published kinky letters, many dealing with spanking, for years. After years one of the readers wrote in that some of those letters dealing with runaway girls being spanked, many of which had been published in the past, had actually been written by his elderly grandmother.