Don't Build Expertise on DK DPS
Since you hitting from behind, there is a small chance to get "Dodged" by bosses on raids.
Well i made a test on Heroic Dummy and these are the results:
Expertise Time Prob Dodge Hits Dodges Normal Crit Glancing
23 0:32:32 0,287% 697 2 265 250 180
17 0:30:49 2,388% 670 16 244 237 173
10 0:30:49 3,075% 683 21 230 252 180
Whatever is your expertise, while shld be min 5 exp (from talents), build anymore of that is just waste of any other valuable stat with real output dps. If u get Dodged by boss, u will need to only redo de hability. Also de Probability to get Dodged 3 times in a row with pleague strike shld be around 0,000029% wich will occur once in 34,843 cases... don't worth it, just re hit the ability.