Are uninsulated steam pipes behind walls and touching beams a fire hazard?
Do you know the temperature of steam pipes? Do you know the ignition point of wood?

Some say the "ignition" point of wood is 451 deg F. Most say around 570 deg F. Either way, the surface temperature of steam pipes will NEVER even get close to that. Typically upwards of 250 deg F. I think you can trust your plumber on this one.