You can always inquire with each vendor directly but, from personal experience, I know Whiteflash will give you a discount as a PS member, as applicable.
I think most offer some type of PS discount that is reflected in backwire price and you can see it on the diamond search page, it''usually marked with an *..
Don''t put too much stock into the "discount" aspect. The bottom line prices are the ones to compare. And as someone already said, when you use the search by cut price search, the final prices are usually given there.