The SV shot priority is:
Kill Shot > Black Arrow > Explosive Shot > Aimed Show > Serpent Sting > Steady Shot
I wouldn't stick to this rigidly though, at least when it comes to serpent sting. It depends a lot on your playstyle, how attentive you are, and whether you have it glyphed or not.
If you glyph serpent sting, the duration becomes 21s (15s base + 6s from glyph). This is 3s shorter than Black Arrow (24s CD) or 2GCDs.
As such, you can cast Black Arrow (1GCD) cast your next high-priority shot (1GCD) and then cast serpent sting, and then by the time Black Arrow is ready to cast again, it will be time to cast serpent sting again. This way, you can avoid having to watch timers or debuff frames when it comes to keeping serpent sting up, as you always cast it just after Black Arrow.
I use this macro to make this even simpler:
Code:
#showtooltip
/castsequence reset=22 Black Arrow, Serpent Sting
Stick it on your action bars and just press it twice instead of once, with an explosive/aimed shot in the middle if they are available. Works a treat if you aren't fussed about watching bars, or want to keep a minimalist UI.