Generally, poisons such as Instant Poison VI provide rogues with more damage increase then dense sharpening stones. However, certain mobs in MC are immune to poison. Generally mobs in MC are either Humanoids, Giants, Elementals or Beasts. The only type that is immune to poison are elementals, but elementals is also the most frequent type of creatures found in the instance.

You may have asked yourself if it would be wise to switch from poisons to sharpening stones depending on the type of mob that is being fought. Altough it would be possible to switch for each and every mob, it would also be very expensive since the trash mobs are a mix of elementals and other types. But it would still be possible to switch from time to time to optimize dps. The main goal is to use each type of weapon enhancment as long as possible, up to the maximum duration that you can get (usually 30 min). Poison can be replaced with sharpening stones if needed since the cost of poisons is significantly lower then sharpening stones.

The list of immune mobs should be something like this:

Immune:

Fire Lord
Lava Anhiliator
Lava Surger
Baron Geddon
Garr
Firewalker
Flameguard
Ragnaros

 
Not immune:

All Core hounds
Molten Giant
Molten Destroyer
Lucifron & adds
Magmadar
Gehennas & adds
Shazzrah (not meleeable)
Sulfuron Harbingger & adds
Golemagg
Majordomo Executus & adds

Considering the immunities, a typical Molten core raid would be somthing like this:

Start
0:00 Trash mobs, Sharpening Stone
0:20 Corehound packs before Lucifron, switch to poison
0:30 Lucifron, poison still on
0:40 Magmadar, poison still on
0:50 Trash mobs, sharpening stone
1:10 Gehennas, switch to poison
1:15 Trash mobs, sharpening stone
1:25 Garr, stone should still be up
1:35 Lava packs, stone all the way
1:45 Baron, poison time, deadly poison since you have to go out-in all the time
1:55 Lava packs, sharpening
2:05 Shazz, w00t go ranged DPS!
2:25 Laval packs, stone will run out, switch to poison
2:35 Sulfuron, poison
2:45 Lava packs, sharpening
3:00 Golemagg, poison (deadly)
3:15 Majordomo, instant poison
3:30 Rag… Sharpening stones …
Phat lewtz, End.