I wait until the ground-temperature is also warm.
You want to make sure that you're in an area that does not use pesticides on the grass, and that has few-to-no delicious insects that your tiny friend might munch. You also want to be aware of other, larger predators -- cats, dogs, birds of prey -- and the potential of an escapee-hedgehog. They're shockingly fast, so no turning your back or your tiny friend will disappear!
Yes, they can pick up mites from the outdoors. I don't know about fleas, but I ended up just doing preventative Revolution anti-mite treatment.