There are some instances where an enemy is completely paralyzed and death is inevitable - it's just a matter of who gets the kill. However, most kills (and this is especially true during laning) involve a lot of uncertainty. If you hesitate for a half second or wait an instant for your carry to get the last hit, your opponent could flash away, Soraka or Shen could ult, the enemy jungler could arrive, your opponent could pop a red elixir.... the list goes on. There are so many ways to narrowly avoid death in LoL. For this reason, I think any hesitation should be discouraged. In general, my mindset when playing LoL is, if there is an opportunity, make the kill happen through any means necessary, even when I'm playing Taric.
I also agree with some of the other posters who say you want the wealth spread over your team. One powerful champion can be CC'd and nuked down. A powerful team is unstoppable.