It's certainly dangerous with regard to cognitive bias, but I have backing from an excellent doctor who has noted my results experimentally, so I am convinced of what I'm saying.
edit: I would also stress that a lot of things that many of my former doctors were skeptical about were mere symptom reports. I think I would know when my intestines hurt, yet some doctors acted like this was malingering, or a "panic attack" when really I have food allergies (but they wouldn't know that since they gave up from the get go).