Evaluate based on your past experience
Which ever way you want it is fine. But it MUST be labeled.
Most people mount the switch as if were a trim wheel; i.e. push the top for down trim as if you were rolling a trim wheel forward.
But again, the choice is yours. Whatever you are most comfortable with.
If the switch were to be mounted horizontally on a side panel, then pressing the rear-most part of the switch would be up trim, and forward would be down trim, and this would also match the typical directional alignment of a hat switch or with the actual joy stick - easier to remember that way. Mounting on the face of the instrument panel does kind of throw a bit of a twist on things though.
If it were me, I would at least take a minute or two to ask myself (CFIs do that anyway out of habit) what would I instinctively want to do if I were in a stressful situation that required immediate action without really thinking about it too much? I would also rely on past experience and training to help me decide. I.e. if you flew a lot of Cessnas with a trim wheel then it might be worth keeping that same orientation just because you are used to it. If you flew with electric trim then I am willing to bet that you had a trim switch mounted horizontally on the yoke of the aircraft - basically the same situation I described above. if you fly IFR alot, then you may also want to consider some other less obvious things, such as the affect of your hand on the stick as you move your other hand to press the switch on the panel. The typical response is for the pilot to push forward slightly on the stick and/or bank slightly in one direction or the other whenever you have to reach forward to do something on the panel (radios, etc.). Now if you are on autopilot all the time, or only flying VFR, well I suppose you can dismiss such tedious things then.
It's true you would have to run some additional wire to either a side panel or to the joystick hat switch, but I don't think that is inherently more compilcated than mounting it on the panel. I could be wrong.
As Mel says, whatever you do - it must be labled.
I'll be curious about your final decision and also where you ultimately decide to mount the switch.