I always give references (ahem,
http://missy-mariposa.com/reference.htm) but I only take them from what I personally consider a reputable provider. When I ask for references I ask if he is "clean and safe" and if there's any important information that I would need to know before seeing him.
I never PM a lady to ask about an okay if she's not listed as a reference unless something is "off" about the client in question.
An example of what I say when asked for a reference would go like this:
"Hi there! I saw XYZ in MonthYear. He was fun to be around, discreet when arriving, very clean, and I had no problems with him. Have a great time!"