As I understand what is happening:
- First, we have a rating.
- Second, we modify it with an adjective, calling it a risk rating.
- Third, we modify the adjective with another adjective, which is why we hyphenate it, resulting in a low-risk rating.
But, is it ok to not hyphenate it and call it a low risk rating.
My feeling is that you'd call it a low-risk whatever when you have varied descriptors such as: low-risk, low-interest, high-volatility; but when you are only talking about risk ratings, you'd leave out the hyphen. "Our optimal risk rating is a low risk rating." Because, now, the noun has become the risk rating as opposed to the rating. (I mean, cripes, should I hyphenate risk-rating?? So many bleeding hyphens!!!)
(I'm asking because Microsoft Word is giving me a bunch of hyphen suggestions and it's annoying the feck out of me.)