Is fear of violence a rational reason to hide being trans from everyone?

If you fear a genuine threat of violence, such as living in a religiously extremist area or having family members who are so bigoted they've said things like they would kill their own kids (or whoever else) if they came out as gay or trans, then no, that is not an irrational reason to hide being trans.

I don't know what living situation you're experiencing, or what living situation your support person is experiencing, but hiding it or being out as trans is a decision you have to make for yourself based on what you know of the environment you're living in. If you fear violence and the threat is real, it would be in your best interest to keep it hidden until you're able to gain independence and move away from there to a safer location. There are a lot of more liberal and accepting areas of the country and world where a trans person can very freely be trans with little to no worry about violence (perhaps harassment, but not actual violence). If you haven't already, start seeing a gender identity therapist for guidance and advice, even if you can only communicate with one online somewhere. Look for GLBT or transgender-specific support groups in your area for advice from others' perspectives. While it is important that you always be true to yourself, your #1 priority is ensuring your own safety and well being.

Trans people experience violence at much higher rates than that general population. Depending where you live, and general political temperament this threat may be real and relevant, or virtually non-existent.

If you're afraid of violence for being who you are I would suggest moving.

Makes sense, many people hide the fact they're trans because of ridicule.

Yes, it is entirely rational. "T" folks are the members of "LGBT" that are most often the victims of violence, and most often killed. It is possible that this is partly due to a tendency of mtf trans people toward involvement in prostitution and drug deals - both activities with a higher-than-average occurrence of violence. If you avoid such activities, fear of violence is less rational, but still not irrelevant.

If you simply wish to be TREATED as your preferred gender, then fear of violence should be largely irrelevant. (There is always the possibility of being targeted for mugging, but that is true for anyone who is out late in an urban setting.) HOWEVER, if you insist on being the center of attention in any setting, that changes. And if you also desire to HAVE SEX, but you are not anatomically equipped to act as your preferred gender, then you put yourself at risk of violence unless you are completely up front with your prospective partner, from the get-go. If you assume "they already know" then you are putting yourself at risk for violence.

Of course, by being frank with everyone around you that you aspire to a different gender - or by being frank with everyone around you that you were not always as you appear to be - you risk rejection, scorn, and ridicule...but not violence. So fear of violence is not necessarily a reason to hide everything. But it IS a rational reason for deciding what to get involved in and what to avoid, and how forthcoming to be, in various situations.

You might discuss these points with your support person and, if you don't like their answers, seek someone else's help.

Your support person makes a valid point

Maybe not from everyone, but from some people if you suspect they will be hostile, yes.