telling someone to open with MU is just plain bad advice

sorry.
If you open with MU then it ticks totally unbuffed, losing a massive amount of damage. You should open with a 3 hit combo (usually DE, unless you have a warrior you can rely on to get Storm's Eye up in 3 GCDs, which case it should be AE), weaving Internal Release and Blood for Blood, then do SF, then MU.
You should also be looking to get Trick Attack up between Spinning Edge and Shadow Fang, so it buffs both dots.
Also: which dot to put up first if none are ticking is dependent on the fight. Do you have 30 seconds of uninterrupted dps time, then MU first, otherwise, SF is better to do first.