I've been using canned gas for over seven years on two bodies with no apparent ill effects, but I strictly follow the following:
1) Never shake the can. In fact, I position the can on my desk some time before the intended use, and don't move it again - I only move the camera in order to blow on the sensor.
2) Never blow at anything but the sensor, not the mirror box, not the mirror or pentaprism.
3) Only blow in short intermittent blasts with a second or two between blasts - this lessens the chance of the refrigerant effect.
I'm not sure how one would "freeze" a sensor considering that all the materials are already frozen (solids). If one aimed at one spot and held the blast continuously at that spot, it could be possible to induce differential temperature stresses. But in truth, the sensor is so small that this would actually difficult to achieve with the amount of movement one normally exhibits when hold the camera body.
In any event, there are countless users that routinely subject their camera bodies to ambient temperatures that are well below what the manufactures recommend in their manuals - with no apparent harmful effects.
Like any operation involving a fine instrument, one must exercise a reasonable amount of caution and intelligence in the procedure.
If the refrigerant doesn't remove the spots, a wet cleaning is in order.
In any event, I will continue to use this method, and don't expect any terrible things to happen.