Something I didn't quite understand in your post:
Let's say I have a task in OmniFocus e.g. "Eat Lunch" and I have the same task in iCal e.g. "Eat Lunch" (and the OmniFocus context and iCal calendar are linked in OmniFocus preferences). That is what you are referring to? In this case, you would recommend that I delete one or the other before syncing?
Or should the iCal task list be free of ALL tasks?
I'll edit that to make the text less confusing.