I'd say if you're SU + CR your MAIN units, then definitely do that. RC trading is for those extra Gold+ units you'll never use (like Stella lol). The RC mechanic isn't going to go away and since putting a rotation in would mean it's practically inaccessible without being a whale, you likely have all the time in the world (well, until the game goes down someday) to get enough to get the units you want. Dark Knight looks useful, but not nearly enough to go crazy selling every Gold+ unit you have that you don't use frequently.