Hasso, I personally would not be shy using a Yamaha dealer for routine maintenance on a Triumph. Bikes are bikes as far as tires, oil, filters, chains etc go.
If you have a Triumph or Haynes service manual to give them it would probably help with torque settings and other procedures.
Valve check and shimming would be another story. There are an awful lot of bike brands out there using shim under bucket setup.
Ask if you can hang around and watch him do the service.....everytime I did this I learned something about my bikes that I never knew.
