Comma with "as" when it means because.

Is a comma normally used before "as" when it is used in the same sense as "because"? By standard grammar rules, a comma is not needed since "as" is a subordinating conjunction, but that is usually when it's used to mean "at the same time" (e.g. "I jumped out of my skin as lightning struck.").

But sometimes it feels like there should be a comma, such as in the following:

"You should always check the credentials of the candidate, as there are a lot of liars and frauds about."

Are there any specific grammar rules regarding this?
Thank you.