Most of the time using Soul Fire during metamorphosis isn't going to be worth it. If you are at high fury, have a lot of very strong cooldowns active, or are under 25% you will probably get a dps increase out of using soulfire in Meta. If none of these are the case I would suggest mainly using MC procs during extra Int procs where meta isn't already being used. MC stacks can be treated as another resource because they stack up to 10 and have a long (30s) window where they can be refreshed, so you don't want to just use them as they happen for optimal damage.