I also use a metal oil pan, on my wooden deck. Whichever cooker I'm using (WSM or Weber kettle)is always on the oil pan. And I always light the chimney inside the kettle (remove the cooking grate and set the chimney on the charcoal grate) so I don't have to worry about hot coals falling out.
In my experience, any piece of charcoal small enough to fall out through the WSM's bottom vents won't cause any problems if it lands on the oil pan. The metal conducts the heat away so quickly that the wood underneath doesn't even get scorched. But Les is right, beware of leaves, mulch, etc. that may accumulate underneath a deck. I raked a lot of leaves out from under mine - and even a tiny coal would have been enough to set them on fire.