Do You Refer People for Jobs? In Person, On Sites like Blue Chip Expert?

Referring people for a job opening is easy if you are confident that they will deliver.
More often than not things will not be so clear cut.
It might be that you are not sure how strong the hiring company is.
The person and the business might not be the best match or you are not 100% sure of their reliability.
How do you decide when you find yourself in this gray area?