On my 2003 Silverado I changed mine to unlock when I pull the key out of the ignition. At night I've had panhandlers at convenience stores walk up beside my truck waiting for me to get out to ask for money. The first time it scared the cr@p out of me because I didn't see him until I opened the door and swung my legs out to stand up.
After that I reset it to unlock when I remove the key from the ignition and always check my side mirrors before pulling the key out. I was concerned someone who wanted to rob me might want to surprise me by yanking the door open when I put it in park before I am ready to get out.