Nah, normaly I'm already at the spawn point before using it being the closest add to me, tho I never use it once the add is not spawned yet (so it doesn't misstarget some other) I believe it targets the closest add and stuns it, therefore I never had any targeting issue. Tho people can use /targetenemy, I believe it works as fine.

Yes Phalk I actually have the same issue as you. My combat MH is a 391 heroic too unlike my Assasination MH 378 dagger. I can probably equal my Assasination DPS on single target with good RNG, but not overtake it by the 1.5k dps that Shadowcraft suggests during a raid. I thought it was just down to practice but I gave combat a good go. Now I am back to Assasination. I would swap back for H Rag though since the expertise is needed and Combat would out-dps Assasination on that fight for sure.

I also find the Assasination rotation much more natural and flowing, but that is a personal thing. I find with combat you have to tunnel much more to make sure you hit all the right timings etc.

Yes, latency might be something to do with it if you are energy capping during AR or Heroism etc.

Assassination isn't really a wash for Heroic Rag. It's quite good for P4 even. The times in which combat is strongest on Ragnaros are Phases 1-3, with Phase 4 making Killing Spree unusable at times. Assassination on the other hand does rather well in Phase 4 due to the execute phase.
If you feel you play better as assassination or simply prefer assassination, there's really no reason you shouldn't play it for the fight. While Blade Flurry is nice for the Sons, it's not a necessity. In the end it's an execution fight more than anything.

It actually depends on how your raid handles rag p4, since the boss usually faces towards the raid, rendering murderous intent pretty much useless. If you're pushing for one meteor and are having problems getting to the point where he's not casting anything anymore, you should give assassination a try, espacially since you are able to support in AoEing the seeds aswell.

Not to forget that you can hand out a strong stun via Vanish-CS to leave a single add alone in the second intermission to nuke the hell out of the scions before rag emerges again. I also enjoy sprints cooldown reset via restless blades in p4, making rogues one of the favorite classes to extinguish the dreadflames.

Using BF on the scions isn't that big a deal if you're pushing for one meteor though, because if the raid isn't able to kill them both before rag emerges again without major cd usage, you're most likely not getting him down to 10% in time anyway.

As well as a Revealing Strike - Kidney Shot for added control. I'd counter that some raids (like mine) have one scion alive going into P3, and the rogues' sole job is to use BF to cleave off Ragnaros onto the scion, which can be tremendous damage when cooldowns, potion, bloodlust and shattering throws are all used simultaneously. We haven't really had a problem pushing one meteor either.

After crawling through the various posts, I saw that when fighting Domo in scorpion form there is a place just off the pincher that you can stand to avoid being parried but still be available to damage share on the flame scythe.

Is there a place to stand when fighting Rag that will avoid all the parries?

I know there has been discussion on points in cuop de grace vs ruthlessness, my question is how is that 1 point that is being debated in comparison to the 10 agi one would get from not having to use assain's step ench due to putting a point in quickening?

I think you will find that the discussion you are referring to has been as between Lethality and Coup de Gras rather than Ruthlessness and Coup de Gras. A talent point in any of these talents (Lethality, Coup de Gras and Ruthlessness) will be at least two to four times as good from a DPS perspective as a point in Quickening + 35 agi on boots instead.
Putting points in Quickening is a DPS loss - though if you elected to put two talents in here for 16% faster move speed and 20% more heals on a fight like Rag HC that might be understandable even though it will be a DPS loss.

I didn't see it mentioned anywhere in the original post, but from my experience (not much as rogue), blade flurry damage on Rhyolith is reduced by armor stacks on the legs, so until the armor is low enough, it will do extremely little damage, so the line "makes him walk in a straight line" isn't quite true.