i usually go for the heavy first then cloak after the backstab cause heavys usually get alerted when their trusty sidekick (mr. medic) usually kills off from a backstab
To hard to kill the medic they are always running around, I just walk up to the heavy's side and blast him with my minigun, usually takes care of him...
When I'm spying, I try to wait as long as I can before backstabbing the heavy. It's just so awesome to see a medic uber himself, realize his heavy is dead, then run away.