I agree that Serpent Sting should be higher up in your shot rotation. In my opinion, SS should come before anything if you're specced into Noxious Stings. Here's my shot rotation:
Serpent Sting
Black Arrow
Explosive
Multi
Steady
Steady
Rinse and repeat, being sure to keep SS up at all times.
L&L:
Explosive
.5 sec
Explosive
.5 sec
Explosive
It's a little better (not to mention more mana efficient) to just wait the .5 sec after the GCD to shoot your next Explosive shot than to fill it in with something else, imo.
