Thought you might be interested to know: in the latest release (27), the 'pick nth instance' system condition allows for a good way of working around the 'collision picking' issue. The problem usually is in the event 'Sprite collides with Sprite' (both objects the same), how do you make actions to affect just one of them?
Pick nth (which is 0 based, like everything else) in subevents can solve this:
<img src="http://www.scirra.com/labs/collisionpick.png">
Hurray, no more "how do I pick that instance" problems