I wouldn't say I'm an expert on DK tanking but every tree seems to be good enough for 5 man content.
I just started tanking on my Blood DK he's at 75 and I have just the crafted armor available to me (a mix of cobalt and tempered saronite.) I managed to tank UK, Nexxus, and AN on normal the other night without trouble. The healer complained of being bored on several fights.
Just make sure you are Def capped at 540 and have 26+K HP and you should do fine in Heroics with a good group.
Check here fro a good starting point on specs