If by "epic jungle mobs", you mean the Ancient Golem and Lizard Elder (the ones with the colored stones floating around them), you should always get them. They give buffs to the champion that kills them. Ancient Golem generally goes to your AP carry for increased mana regeneration and cooldown reduction. Lizard Elder, on the hand, is generally more efficient on either your bruiser or AD carry since it grants on-hit slows and burn effects.
If by "epic jungle mobs", you mean Baron Nashor and the Dragon (the mobs along the river), then it depends. Dragon should taken as soon as it is safe to do so, as it grants bonus gold to everyone on the team that kills it. Baron Nashor, on the other hand, must be killed carefully. It is a very strong monster will often take upwards of three champions to kill (sometimes requiring a team effort). Due to this, Baron is often contested and an ace at Baron usually results in a loss for the aced team. Killing Baron gives a team buff that increases damage dealt.
In summation:
Ancient Golem (Big rock dude with floating blue stones): Gives buff that dramatically increases mana regeneration and cooldown reduction.
Lizard Elder (Giant lizard thing with floating red stones): Gives buff that makes basic attacks slow and deal burn damage (this damage is periodic and true).
Dragon (... kinda self-explanatory): Gives 190 gold to the team that kills it.
Baron Nashor (giant purple worm of DOOM): Gives 300 gold to the team that kills it (as well as 25 gold to the champion that gets the last hit) along with a buff that increases AD and AP as well as health and mana regeneration.
Credit to Shykin for correcting me on dragon gold and providing additional information on Baron.