John,

The client will be much more disappointed once they move in and have to perform possible repairs just because you turned on the equipment to show them it operated.

That alone does not prevent a visual inspection of the equipment to look for other defects or issues.

As Jim said, at those lower exterior temperatures your not going to determine if the equipment is cooling properly anyway.

Sometimes after you have explained all the reasons to the client, the agent will go behind you and turn on the thermostat and take the client outside of say, See its running.

rick