I mount to 4ply board, and over mat around the print edge (1/2") with matching white 4ply board. Check out www.SuperiorArchivalMats.com and read the test done on mounting on Alpha Rag Artcare board. I suggest using the "White" 4ply. This is the best mat board in the whole world. Period.
That website has it for sorta expensive, but you can find Artcare board at most framing stores for cheeper.