So going from the statistics with Harj (normal or heroic)
https://www.warcraftlogs.com/statist...ntinfo=Talents either MG or WiA are almost equal choices. So then pick your favourite/whatever you are best with. It also depends on your group's tactics and how far they make you move.
It can vary but if you play with MG on a multi-target fight where you are focusing boss/do not have to DPS very short lived adds (something like Harj murlocs) then just tag them for WoC and souls (usually with Corruption/Seed if they are stacked as Agony doesn't have chance to warm up) and tunnel the main target and drain it.
If an add lives a little longer but you still do not have to focus it you can start applying Agony as well for extra shards. Icy adds on Spellblade was a good example of this. Maiden on Fallen Avatar as well assuming you can get away with one DoT on her and use it to increase your single target damage on the Avatar. On heroic personally I prefer WiA to deal with the incresed movement but on normal I'll tend to use MG still.
Once you have to start moving more (think Mistress) or have a lot of adds to tag (Mistress/Desolate) or even two constant targets (DI) then it's unlikely you can find enough time to drain for MG to be worth taking (unless you really really need to focus single target damage but for my guild we get overrun with adds). MG is not just about draining the UAs, all of our DoTs hit pretty hard now so you'll want a decent uptime regardless. If you cannot that is when usually you will want to take WiA.
On (norm/hc) Mistress, DI and Desolate WiA is really strong. DI is personally the only boss I take SL on in the whole raid as like you said before globals are really important for multidotting. The more agonies you have rolling the more UAs you will need to use up. Again go through the statistics page above for each fight and you can see which talents tend to do better. However, that is for overall damage and if you need to focus on one part then change your gameplan.
Like Mineral said drain sniping adds is really important in order to pump in more UAs. Also don't forget draining entirely when you have nothing else to do as it will still give Rend Soul procs (assuming you have the 4th golden).
Also have a read of this site
http://lockonestopshop.com/#!/Affliction/Introduction if you haven't yet.
Does this all make sense to you?