You should cast it before the fight starts. You then get a buff that lasts 60 minutes that increases your damage of the named spells. If you die then you need to redo it.
Best way to find out what pet gives is well to sacrifice them and check the Command Demon ability. The comments on this
Wowhead page also lists them. I tend to run with Shadow Bulwark from Voidy for a personal cooldown however if we are failing on interrupts then I will change over to the Felhunter. Basically it just depends on what the group needs.
Is GoSac optimal? Or the pet you summon? Yeah GoSac goes with Charred Remains but you may need a certain number of tier pieces/stats/decent procs to rock those talents. Perhaps on a very high movement fight a pet may be better or if you are AoEing a lot but I personally have not come across this yet.