Sometimes it's difficult to know whether or not something is important enough to share with a massage therapist. One reason is ignorance. Sometimes you just don't think some aspect of your history is relevant to a massage. Another reason is embarrassment. There are societal stigmas that come with health issues. It's not very easy to share that you are being treated for a mental illness. But there are types of massage that you shouldn't receive if you're on certain medications, or if you have certain health conditions.
For example, did you know that deep tissue massage is not recommended for someone on steroids? Your connective tissue could be compromised if you've been on steroids for a long time. A deep tissue massage may very well cause you to wake up the next day with bruises all over. If your doctor has you on steroids for a while, say, longer than two weeks, you should definitely tell your massage therapist.
There's also the issue of your muscles: they remember things. You could have been in a car crash five years ago and gone through the healing process, but find your back still tensing up when you get in a car. The same way your muscles remember how to ride a bike after twenty years, is the same way they remember a trauma that you experienced. And sometimes, dealing with the emotional ramifications of that trauma (using Swedish relaxing techniques as opposed to deep tissue) is the more effective way to go.
Then there's compensation. If your muscles protect a certain area by tensing up for long enough, another part of your body may take over the stress and then eventually start to hurt. So you should tell your therapist about the injury to your hip that happened ten years ago because it might very well be the source of your neck pain today.
In short, the best way to make sure that your therapist takes care of you properly is to tell her exactly what you've been through. If you're wondering if she needs to know something, err on the side of too much information. Or an even better way of figuring it out is to tell your therapist what you would tell your primary care physician. Your massage therapist may not be able to cure your diabetes, but she can have an effect on it. Proper knowledge determines whether it's positive or negative.