This is my all-purpose questing/dungeon setup:
http://beta.wowdb.com/talent-calculator#Cr5l
Writhe isn't worth it for dungeons or questing since few mobs live long enough for it to get much past 10 stacks, shard return off drain soul is just too good to pass on and the damage isn't bad either and helps trigger seeds if you dont have shards. You can make a case for haunt but functionally DS is just superior in similar roles.
Level 30 is pretty much preference, contagion currently doesn't apply to UA damage if it doesn't already have a UA ticking so I feel that reduces the advantage significantly, but a case can definitely be made for it. I find the convenience of AC too good to pass, especially since I use effigy for almost everything. Not even gonna mention the last one.
Demon Skin and Sow are no-brainers for leveling. If you like running around watching things slowly tick away and then you can take siphon+sac. If ever want to do real aoe damage though you're gonna need Sow.
Grimoire row... I like aoe padding in dungeons so I just use sac for leveling even though I usually have VW out anyways. When a queue pops I just sac my pet. If you want maximum quest deeps then Serv is almost certainly better.
As said above I tend to have effigy for everything since I find the ~40% dot, shard and 100% CI increases to just be absolutely huge in any single target situation. I'm rarely shard starved questing due to DrainSoul and trying to fight ST bosses/rares without effigy is pretty miserable. Conduit definitely has it's uses, but if you have 3-4 agony's out you tend to have enough shard fuel for SoC spam regardless of whether you have conduit or not. Effigy has additional utility with sac/sow since you can drop it in an AoE pack on a priority target and get a lot more damage on said target since effigy copies 40% of each seed/sac, obviously has the disadvantage of melee range.
I've quested a lot and done plenty of dungeons so I'm a pretty firm believer in my setup.