Keffudle, I would not evacuate based on a warning light, if there are no other signs of fire. You know that it might be overripe fruit or a burst can of deodorant causing the warning. I sure as hell would declare an emergency and land ASAP. But once on the ground and still in tact, I would want more signs of a fire (which would have been raging for more than 20 minutes!), before I evacuate. On the other hand, I would NOT park anywhere near a terminal until cleared by the fire fighters.
You ask for signs of fire...well...the ABSENCE of any further signs is a sign.
If the cargo hold is so well insulated that you don't see any signs after 20 minutes of fire, then you might as well wait for steps. Just my humble, untrained in firefighting, opinion.
Regarding the evacuation itself, I'm not entirely sure that it's such a mortal danger on a lowish 737.