On an encounter like council of elders you prioritize soulburn+soulswap over haunt for shard utilization and you keep all 3 dots on all 4 bosses which cost you 3 GCD every 14 seconds; this allow you to grasp the empowered target; if you have spare soul shards which you should have, you haunt the empowered target.
Forgive my bad english but i hope you got the point.
I just use shards to SB:SS the targets other than the main target. Keeping corruption up on all of them means more Nightfall procs = more shards. Also drain soul the Marli add as it dies to get full shards then fully dot everything up again. Don't bother with MG other than on the main target.
I keep all 3 up. I don't usually concentrate on MG channeling, just making sure all dots are ticking on everything, including the spirit. I'll usually DS the spirit to get all shards back, then SB:SS all 4, concluding with the main target (Sul > Empowered) so I can get as much MG channeling on it as possible before I go back to the next spirit. Hope that helps!
Personally i refresh with SB:SS every time corruption is about to fall off. Earlier may result in too few soul shards to keep them up and after corruption falls off you'll have no more nightfall procs.