It depends on the paper used for the Pt/pd print. I usually leave a significant border around the print to show the hand coating artifact. So, I would like the paper and the cover mat to be similar in tone.
I just compared several platinum prints made on Arches Platine and Fabriano Artistico White, developed in potassium oxalate and ammonium citrate, to a sample pack from Lodima Archival Materials.
I am not sure how "Natural White" from Light Impressions compares, but I find the one from Bainbridge to be a little too green for my tastes. Natural White is actually classified as a yellow-green on the Nielsen and Bainbridge website, whereas White and Pearl White are considered neutral whites.
For platinum prints I would recommend using "Pearl White." It is just off-white enough to not clash with the soft, warm whites in the prints and it isn't as dark or yellow as something like Antique White, Cream and Ivory.
". . . photographing as a two-way act of respect. Respect for the medium and letting it do what it does best- describe. And respect for the subject in describing it as it is. A photograph must be responsible to both."-- Garry Winogrand
I do it two ways. One, I use 100% cotton rag pure white 4ply. I use the same board for matt and frame. I leave a 10 mm open boarder to let the black or brown edge show. the second way is to cut the print just like a Azo print and leave a 15mm open boarder to sign and title the print. I like cutting the prints because there is no influence from the darker boarder on how the print should look.
"Capturing an image is only one step of the long chain of events to create a beautiful Photograph” See my updated website: mandersenphotography.com