This is a strange question. Yes crawl under your house to see if there is a leak. Bring a head lamp, a p&s camera, and a strong stomach (who knows what you'll find down there).
Uhh yea crawl under there. I go under all the time to open/close the outside vents.
Ohh also there shouldn't be exposed dirt under your house. There should be plastic laid down so that you don't have a moisture issue that will cause mold. Our house was inspected and everything checked out fine. But when we went under the house we realized that there was no plastic and that there were some wet spots. The wet spots were from the down spouts of the gutters facing in the wrong direction, so we fixed that. But they had put in insulation under the floor joists. We hired someone to come out and he fixed it all and laid plastic for around $500. So that's something you may want to look into if you do have exposed dirt.
We close ours in the winter when it gets really cold. It saves on our heating bill and helps to keep our pipes from freezing. They also recommend it when it gets extremely humid outside.
We have wood floors and it gets too humid under our house so we close our vents and have a dehumidifier so that our floors won't cup or buckle.
My DH is a plumber, so he's always down there to work on stuff. Me, on the other hand, - I agree with housegeek, you couldn't pay me to go down there. We also have to run a dehumidifer in the winter, takes out the moisture & the musty smell.
Maybe our crawlspace is different but it's completely sealed up. Our vents have mesh screens and the doorway/crawlway is sealed with a door. Nothing could possibly get in there, besides bugs/mice. We have a pest control contract so we really don't have an issue with those either. I guess maybe I should be more afraid than I am??
LOL well what can I say I'm a tomboy. We don't personally have a crawlspace but I've been in many of them for one of my old summer jobs installing ductwork in old homes with tight attic and crawlspaces. Lots of dead or living mice, squirrels, and even a raccoon. Also one of the previous owners of a house had let their cats use the crawlspace as a litter box. That smelled nice in 100 degree heat.
I have been in the crawl space under the sun room (converted porch). It is not my favorite place but it took both DH and I to be there to run the wiring for the surround sound. DH had the expertise but needed another set of hands to hold certain things in place.