I have a safety on my Shield and I appreciate having it. I like the little extra security of being able to engage the safety when I'm handling the gun. However, I do not carry it with the safety engaged. When it goes into the holster the safety is off and remains that way.
The Shield's safety lever is small and flat which makes it carry/conceal easier, but I wouldn't want to have to thumb it off in an adrenalin filled moment. However, that configuration, along with the fact that it engages pretty firmly, gives me confidence that nothing is going to engage it inadvertently while it is in the holster.