Using Berserk at the beginning of a fight is great since usually any proc abilities proc immediately at the start of the fight, making those cheap Shreds hit extra hard. I usually use it after I deplete my energy bar though, and then hit TF, then go Berserk. Before, this pretty guaranteed that I would rip threat off the tank, but thanks to the threat increase that is no longer the case. One thing to keep in mind is if the boss has any vulnerable states like Warlord Zon'ozz or Hagara, or burst phases like the Tendon in Spine of Deathwing, saving it for then might be a better idea.

That's generally what you want to do. Before the tank threat buff, that would have been a bit dangerous (I remember pulling aggro in 5-mans when I would do that), but it's generally a dps increase to use your cooldowns on the start of the fight, unless there is some mechanic that would prevent you from going all out on dps at the start of a fight.

"I usually use it after I deplete my energy bar though, and then hit TF, then go Berserk."

Exactly!, good job sir. Here, have a cookie :)