In general, you will want the sensors to be as close to the expected cause as possible while not in the way of operation. Monnit sensors comes with 2 variants generally, one with probe and one without probe.
For the sensors that come without, we suggest placing the sensor itself on the equipment. You will want to have as solid a connection as possible so that there is no additional vibration added between the sensor and the machine. Straps or Ties are typically better than Sticky tape or similar.
For sensors that come with the probe, mount the sensors where it is stable and allow the probe to be attached to the expected cause or site where you want to measure the parameter.