Some of the time, iSync does let you see what is being changed on your contacts and asks you which contact you want to select as the correct one. The choice I usually get is between the computer and .Mac. Then it asks you if you want to sync again to make the changes after you have done this. That occurs at the end of the process and doesn't seem to occur every time.
I only make modifications from my Powerbook and then sync with .Mac so I always select powerbook when given this choice.I select .Mac if I am using my iMac. (Now that I really think about it, this may only happen when I am using our iMac. I think my preference was set that my powerbook is always the overriding choice. So it doesn't ask me.)