I take it that you play a DW frost dk. For pve it's recommended to put razorice on MH and fallen crusader on offhand. Razorice is expose armor + dmg increase, fallen crusader is heal + stacking strength buff. In pvp most tend to play with fallen crusader on MH and razorice on offhand. That is for solo play. I prefer razorice on MH however in pvp.

You never ever want to reforge mastery into haste for pvp. Everything should be reforged into mastery.