You use rip when
1) the DoT will be able to do its full damage (bosses in instances/raids, some elite mobs while soloing)
2) you do not want to lose unnecessary energy (ferocious bite converts all the energy you have left into damage, meaning you have 0 energy after a ferocious bite)
3) a bit related to 1): the target has high armor (usually bosses, elites, and paladins/druids/warriors in PvP)
For now you should probably use FB, but you can also try ripping and moving on to the next target instead of waiting for your current target to die.
If you are really mowing down everything in sight, using FB as a finisher kills your current target fast, but also leaves you with 0 energy, which might be counter-productive when grinding mobs that are close together.
Pounce, especially when you have brutal impact, gives you a nice amount of time where you are hitting the target but the target isn't hitting you. This makes it the default opener for any mob you think might put up a fight.
You'll rarely want to use ravage unless you know it's gonna do massive damage, for example against caster mobs that are green to you (it really rips them apart)