The pieces of my medical history that are likely to affect how the person and I interact with one another fall into that "necessary information" category for me. This includes things like the fact that I have Asperger's, the fact that certain parts of my body are incredibly ticklish, &c, though the specific pieces of information will vary from partner to partner. YMMV, naturally; I personally expect a great deal of candor (http://maradydd.livejournal.com/51676.html) from those I have sex with, but not everyone operates that way.