To answer the "when" part of the question, it depends on the champion you are using as a jungler. Some are very powerful at level 2(shaco, sin, shyv) whereas some champs need to be level 4 before they can output a viable amount of damage(noc, mundo).
I believe that when ganking bot with a damage type jungler, try to let the AD carry get the kills, but elsewhere, it doesnt matter as much, since a fed jungler is going to have devastating ganks and do tons of damage.
If you are jungling with a utility champ (naut, ali, maokai) then its better for the lane to get the kill, since you arent there to do massive damage from great items, but rather be a pain in the but to the other team with your excessive amounts of CC. So just get some GP5 items, get a little bit of health, and CC the crap out of the enemy team in team fights later on.