I've had more avionics problems due to corrosion and damp, than for anyother reason. In the U.K I've never had anything overheat, but I also wouldn't turn everything on if the ambient temperature was high.
Like most things in flying, just use a little common-sense, If the weather is cold and miserable, then turning on all the avionics will be a benefit, but during a hot summer when everything will be dry anyway, what's the point.
Horses for courses.