ALPA came out a couple of years ago saying that they would back any crew that declined an airplane because it was too hot. This was after a rash of incidents with not only passengers, but crew members suffering from heat stroke. Passengers pay us to take them from A to B, not cook them. I would say use your best judgement on this. IF you feel that the aircraft is too hot then decline to take it. Think of some little old lady sitting back with a heart condition, do you really want to have the death of someone on your conscience from heat stroke when you were kicking around the idea of not taking the airplane in the first place?