I'd say it depends on the soil in the crawl space. I install a water heater for my daughter awhile back and set it in a pan. Then while exploring how to best run the drain for the pan, I realized that her house is build on old river bed. So, I just ran the drain into the crawl space because you couldn't saturate that ground if you ran water in it for a month. Unless you have that type of soil, I'd suggest you run the pan overflow to the outside. When a PRV pops, it's just a small stream of water anyway, and if the tank goes south, they will leak but not suddenly fall apart. I suppose a moisture alarm in the pan wouldn't hurt, but I would lose sleep over not have one. Now, if the tank is over finished living space, you certainly will want the overflow to either go outside or into a drain.