This has been bugging me for a while. I've seen streams of high ELO junglers, and it seems like they always use smite as the killing blow on the jungle creeps. Why do they do that? I mean, damage is damage, and using it early could save you a few second on cooldown. Why don't they start fighting the jungle minion by smiting them?
SaintVicious, jungler for the CLG.NA team, once said it on stream. If you are certain that you are not going to be counterjungled, then sure, smite right away. However, circumstances can quickly change while you are attacking the creep (assuming smite is used on objectives like buffs).
Situation #1: You are jungling but leave base a little late. Like usual you head to Red and you feel confident so tell your team you don't need their help leashing it. So you smite right away and begin autoattacking it. In the animation of your last auto-attack, the enemy jungler smites it away from you over the wall, then turns and kills you with the help of the small lizards and the red buff they now have cuz your smite was down.
Situation #2: You have been jungling and cleared the jungle already. As typical, you get ready to feed your AP Carry the blue buff. While, waiting for the AP Carry to deal the finishing blow, the whole enemy team comes charging from river to the Blue led by their jungler. Fortunately this time, you saved your smite, so before the enemy jungler can smite steal it, you secure blue for your team. While not optimal, certainly better than the enemy team having both blue buffs.
Finally, it is good practice as it helps you use smite to secure important objectives like dragon and baron. After all practice makes perfect, and if you say that you always use smite first except for dragon and baron, you won't be as practiced. A few seconds lost on the smite cooldown is hardly noticeable most of the time, but gaining a little more practice on using your smite to secure kills never hurts anyone.
I also smite for the kill. I do this because other people may be roaming to steal the buff. This would ruin the jungle route and would result in a slower jungle clear time causing less ganks causing possible lane loss and so forth. It would also be beneficial for against a stealthed jungler such as Shaco, Eve, and Twitch since they could be there and smite for the kill. Smiting early may also result in the loss of major objectives like baron and dragon. If one is smited early others can come and smite to steal that bonus gold or baron buff. Although smiting early would in fact reduce the cooldown its better to be safe than sorry.